.Net: Fix MistralAI function calling and add image content support (#…

…9844)

### Motivation and Context

Tool messages were being rejected as bad requests

Close #9806 

### Description

<!-- Describe your changes, the overall approach, the underlying design.
These notes will help understanding how your code works. Thanks! -->

### Contribution Checklist

<!-- Before submitting this PR, please make sure: -->

- [ ] The code builds clean without any errors or warnings
- [ ] The PR follows the [SK Contribution
Guidelines](https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md)
and the [pre-submission formatting
script](https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md#development-scripts)
raises no violations
- [ ] All unit tests pass, and I have added new tests where possible
- [ ] I didn't break anyone 😄

dotnet/src/Connectors/Connectors.MistralAI.UnitTests/Client/MistralClientTests.cs

@@ -37,6 +37,24 @@ public void ValidateRequiredArguments()
 #pragma warning restore CS8625 // Cannot convert null literal to non-nullable reference type.
     }
 
+    [Fact]
+    public void ValidateDeserializeChatCompletionMistralChatMessage()
+    {
+        var json = "{\"role\":\"assistant\",\"content\":\"Some response.\",\"tool_calls\":null}";
+
+        MistralChatMessage? deserializedResponse = JsonSerializer.Deserialize<MistralChatMessage>(json);
+        Assert.NotNull(deserializedResponse);
+    }
+
+    [Fact]
+    public void ValidateDeserializeChatCompletionResponse()
+    {
+        var json = "{\"id\":\"aee5e73a5ef241be89cd7d3e9c45089a\",\"object\":\"chat.completion\",\"created\":1732882368,\"model\":\"mistral-large-latest\",\"choices\":[{\"index\":0,\"message\":{\"role\":\"assistant\",\"content\":\"Some response.\",\"tool_calls\":null},\"finish_reason\":\"stop\"}],\"usage\":{\"prompt_tokens\":17,\"total_tokens\":124,\"completion_tokens\":107}}";
+
+        ChatCompletionResponse? deserializedResponse = JsonSerializer.Deserialize<ChatCompletionResponse>(json);
+        Assert.NotNull(deserializedResponse);
+    }
+
     [Fact]
     public async Task ValidateChatMessageRequestAsync()
     {
@@ -62,7 +80,7 @@ public async Task ValidateChatMessageRequestAsync()
         Assert.Equal(0.9, chatRequest.Temperature);
         Assert.Single(chatRequest.Messages);
         Assert.Equal("user", chatRequest.Messages[0].Role);
-        Assert.Equal("What is the best French cheese?", chatRequest.Messages[0].Content);
+        Assert.Equal("What is the best French cheese?", chatRequest.Messages[0].Content?.ToString());
     }
 
     [Fact]
@@ -504,6 +522,31 @@ public void ValidateToMistralChatMessages(string roleLabel, string content)
         Assert.Single(messages);
     }
 
+    [Fact]
+    public void ValidateToMistralChatMessagesWithMultipleContents()
+    {
+        // Arrange
+        using var httpClient = new HttpClient();
+        var client = new MistralClient("mistral-large-latest", httpClient, "key");
+        var chatMessage = new ChatMessageContent()
+        {
+            Role = AuthorRole.User,
+            Items =
+                [
+                new TextContent("What is the weather like in Paris?"),
+                    new ImageContent(new Uri("https://tripfixers.com/wp-content/uploads/2019/11/eiffel-tower-with-snow.jpeg"))
+                ],
+        };
+
+        // Act
+        var messages = client.ToMistralChatMessages(chatMessage, default);
+
+        // Assert
+        Assert.NotNull(messages);
+        Assert.Single(messages);
+        Assert.IsType<List<ContentChunk>>(messages[0].Content);
+    }
+
     [Fact]
     public void ValidateToMistralChatMessagesWithFunctionCallContent()
     {
@@ -544,6 +587,41 @@ public void ValidateToMistralChatMessagesWithFunctionResultContent()
         Assert.Equal(2, messages.Count);
     }
 
+    [Fact]
+    public void ValidateCloneMistralAIPromptExecutionSettings()
+    {
+        // Arrange
+        var settings = new MistralAIPromptExecutionSettings
+        {
+            MaxTokens = 1024,
+            Temperature = 0.9,
+            TopP = 0.9,
+            FrequencyPenalty = 0.9,
+            PresencePenalty = 0.9,
+            Stop = ["stop"],
+            SafePrompt = true,
+            RandomSeed = 123,
+            ResponseFormat = new { format = "json" },
+        };
+
+        // Act
+        var clonedSettings = settings.Clone();
+
+        // Assert
+        Assert.NotNull(clonedSettings);
+        Assert.IsType<MistralAIPromptExecutionSettings>(clonedSettings);
+        var clonedMistralAISettings = clonedSettings as MistralAIPromptExecutionSettings;
+        Assert.Equal(settings.MaxTokens, clonedMistralAISettings!.MaxTokens);
+        Assert.Equal(settings.Temperature, clonedMistralAISettings.Temperature);
+        Assert.Equal(settings.TopP, clonedMistralAISettings.TopP);
+        Assert.Equal(settings.FrequencyPenalty, clonedMistralAISettings.FrequencyPenalty);
+        Assert.Equal(settings.PresencePenalty, clonedMistralAISettings.PresencePenalty);
+        Assert.Equal(settings.Stop, clonedMistralAISettings.Stop);
+        Assert.Equal(settings.SafePrompt, clonedMistralAISettings.SafePrompt);
+        Assert.Equal(settings.RandomSeed, clonedMistralAISettings.RandomSeed);
+        Assert.Equal(settings.ResponseFormat, clonedMistralAISettings.ResponseFormat);
+    }
+
     public sealed class WeatherPlugin
     {
         [KernelFunction]

dotnet/src/Connectors/Connectors.MistralAI.UnitTests/MistralAIPromptExecutionSettingsTests.cs

@@ -1,5 +1,6 @@
 // Copyright (c) Microsoft. All rights reserved.
 
+using System;
 using System.Text.Json;
 using Microsoft.SemanticKernel;
 using Microsoft.SemanticKernel.Connectors.MistralAI;
@@ -68,4 +69,44 @@ public void FromExecutionSettingsWhenSerializedHasPropertiesShouldPopulateSpecia
         Assert.True(MistralExecutionSettings.SafePrompt);
         Assert.Equal(123, MistralExecutionSettings.RandomSeed);
     }
+
+    [Fact]
+    public void FreezeShouldPreventPropertyModification()
+    {
+        // Arrange  
+        var settings = new MistralAIPromptExecutionSettings
+        {
+            Temperature = 0.7,
+            TopP = 1,
+            MaxTokens = 100,
+            SafePrompt = false,
+            Stop = ["foo", "bar"]
+        };
+
+        // Act  
+        settings.Freeze();
+
+        // Assert  
+        // Try to modify a property after freezing  
+        Assert.Throws<InvalidOperationException>(() => settings.Temperature = 0.8);
+        Assert.Throws<InvalidOperationException>(() => settings.TopP = 0.9);
+        Assert.Throws<InvalidOperationException>(() => settings.MaxTokens = 50);
+        Assert.Throws<InvalidOperationException>(() => settings.SafePrompt = true);
+        Assert.Throws<NotSupportedException>(() => settings.Stop.Add("baz"));
+    }
+
+    [Fact]
+    public void FreezeShouldNotAllowMultipleFreezes()
+    {
+        // Arrange  
+        var settings = new MistralAIPromptExecutionSettings();
+        settings.Freeze(); // First freeze  
+
+        // Act  
+        settings.Freeze(); // Second freeze (should not throw)  
+
+        // Assert  
+        // No exception should be thrown  
+        Assert.True(settings.IsFrozen); // Assuming IsFrozen is a property indicating the freeze state  
+    }
 }

dotnet/src/Connectors/Connectors.MistralAI/Client/ChatCompletionRequest.cs

@@ -44,6 +44,22 @@ internal sealed class ChatCompletionRequest
     [JsonIgnore(Condition = JsonIgnoreCondition.WhenWritingNull)]
     public int? RandomSeed { get; set; }
 
+    [JsonPropertyName("response_format")]
+    [JsonIgnore(Condition = JsonIgnoreCondition.WhenWritingNull)]
+    public object? ResponseFormat { get; set; }
+
+    [JsonPropertyName("frequency_penalty")]
+    [JsonIgnore(Condition = JsonIgnoreCondition.WhenWritingNull)]
+    public double? FrequencyPenalty { get; set; }
+
+    [JsonPropertyName("presence_penalty")]
+    [JsonIgnore(Condition = JsonIgnoreCondition.WhenWritingNull)]
+    public double? PresencePenalty { get; set; }
+
+    [JsonPropertyName("stop")]
+    [JsonIgnore(Condition = JsonIgnoreCondition.WhenWritingNull)]
+    public IList<string>? Stop { get; set; }
+
     /// <summary>
     /// Construct an instance of <see cref="ChatCompletionRequest"/>.
     /// </summary>

dotnet/src/Connectors/Connectors.MistralAI/Client/ContentChunk.cs

@@ -0,0 +1,13 @@
+// Copyright (c) Microsoft. All rights reserved.
+
+using System.Text.Json.Serialization;
+
+namespace Microsoft.SemanticKernel.Connectors.MistralAI.Client;
+
+[JsonDerivedType(typeof(TextChunk))]
+[JsonDerivedType(typeof(ImageUrlChunk))]
+internal abstract class ContentChunk(ContentChunkType type)
+{
+    [JsonPropertyName("type")]
+    public string Type { get; set; } = type.ToString();
+}

dotnet/src/Connectors/Connectors.MistralAI/Client/ContentChunkType.cs

@@ -0,0 +1,62 @@
+// Copyright (c) Microsoft. All rights reserved.
+
+using System;
+using System.Diagnostics.CodeAnalysis;
+using System.Text.Json.Serialization;
+
+namespace Microsoft.SemanticKernel.Connectors.MistralAI.Client;
+
+internal readonly struct ContentChunkType : IEquatable<ContentChunkType>
+{
+    public static ContentChunkType Text { get; } = new("text");
+
+    public static ContentChunkType ImageUrl { get; } = new("image_url");
+
+    public string Type { get; }
+
+    /// <summary>
+    /// Creates a new <see cref="ContentChunkType"/> instance with the provided type.
+    /// </summary>
+    /// <param name="type">The label to associate with this <see cref="ContentChunkType"/>.</param>
+    [JsonConstructor]
+    public ContentChunkType(string type)
+    {
+        Verify.NotNullOrWhiteSpace(type, nameof(type));
+        this.Type = type!;
+    }
+
+    /// <summary>
+    /// Returns a value indicating whether two <see cref="ContentChunkType"/> instances are equivalent, as determined by a
+    /// case-insensitive comparison of their labels.
+    /// </summary>
+    /// <param name="left"> the first <see cref="ContentChunkType"/> instance to compare </param>
+    /// <param name="right"> the second <see cref="ContentChunkType"/> instance to compare </param>
+    /// <returns> true if left and right are both null or have equivalent labels; false otherwise </returns>
+    public static bool operator ==(ContentChunkType left, ContentChunkType right)
+        => left.Equals(right);
+
+    /// <summary>
+    /// Returns a value indicating whether two <see cref="ContentChunkType"/> instances are not equivalent, as determined by a
+    /// case-insensitive comparison of their labels.
+    /// </summary>
+    /// <param name="left"> the first <see cref="ContentChunkType"/> instance to compare </param>
+    /// <param name="right"> the second <see cref="ContentChunkType"/> instance to compare </param>
+    /// <returns> false if left and right are both null or have equivalent labels; true otherwise </returns>
+    public static bool operator !=(ContentChunkType left, ContentChunkType right)
+        => !left.Equals(right);
+
+    /// <inheritdoc/>
+    public override bool Equals([NotNullWhen(true)] object? obj)
+        => obj is ContentChunkType otherRole && this == otherRole;
+
+    /// <inheritdoc/>
+    public bool Equals(ContentChunkType other)
+        => string.Equals(this.Type, other.Type, StringComparison.OrdinalIgnoreCase);
+
+    /// <inheritdoc/>
+    public override int GetHashCode()
+        => StringComparer.OrdinalIgnoreCase.GetHashCode(this.Type);
+
+    /// <inheritdoc/>
+    public override string ToString() => this.Type;
+}

dotnet/src/Connectors/Connectors.MistralAI/Client/ImageUrlChunk.cs

@@ -0,0 +1,11 @@
+// Copyright (c) Microsoft. All rights reserved.
+
+using System;
+using System.Text.Json.Serialization;
+
+namespace Microsoft.SemanticKernel.Connectors.MistralAI.Client;
+internal class ImageUrlChunk(Uri imageUrl) : ContentChunk(ContentChunkType.ImageUrl)
+{
+    [JsonPropertyName("image_url")]
+    public string ImageUrl { get; set; } = imageUrl.ToString();
+}

dotnet/src/Connectors/Connectors.MistralAI/Client/MistralChatCompletionChunk.cs

@@ -43,7 +43,7 @@ internal sealed class MistralChatCompletionChunk
 
     internal string? GetRole(int index) => this.Choices?[index]?.Delta?.Role;
 
-    internal string? GetContent(int index) => this.Choices?[index]?.Delta?.Content;
+    internal string? GetContent(int index) => this.Choices?[index]?.Delta?.Content?.ToString();
 
     internal int GetChoiceIndex(int index) => this.Choices?[index]?.Index ?? -1;
 

dotnet/src/Connectors/Connectors.MistralAI/Client/MistralChatMessage.cs

@@ -15,7 +15,16 @@ internal sealed class MistralChatMessage
     public string? Role { get; set; }
 
     [JsonPropertyName("content")]
-    public string? Content { get; set; }
+    [JsonIgnore(Condition = JsonIgnoreCondition.WhenWritingNull)]
+    public object? Content { get; set; }
+
+    [JsonPropertyName("name")]
+    [JsonIgnore(Condition = JsonIgnoreCondition.WhenWritingNull)]
+    public string? Name { get; set; }
+
+    [JsonPropertyName("tool_call_id")]
+    [JsonIgnore(Condition = JsonIgnoreCondition.WhenWritingNull)]
+    public string? ToolCallId { get; set; }
 
     [JsonPropertyName("tool_calls")]
     [JsonIgnore(Condition = JsonIgnoreCondition.WhenWritingNull)]
@@ -27,7 +36,7 @@ internal sealed class MistralChatMessage
     /// <param name="role">If provided must be one of: system, user, assistant</param>
     /// <param name="content">Content of the chat message</param>
     [JsonConstructor]
-    internal MistralChatMessage(string? role, string? content)
+    internal MistralChatMessage(string? role, object? content)
     {
         if (role is not null and not "system" and not "user" and not "assistant" and not "tool")
         {