Remove runtime responses once written

Fix memory leak from build up of `ResponseContext` instances by removing them from the dictionary once used to write the `LambdaTestResponse` to the associated channel.

src/AwsLambdaTestServer/RuntimeHandler.cs

@@ -330,7 +330,11 @@ private async Task CompleteRequestChannelAsync(
         bool isSuccessful,
         CancellationToken cancellationToken)
     {
-        var context = _responses[awsRequestId];
+        if (!_responses.TryRemove(awsRequestId, out var context))
+        {
+            throw new InvalidOperationException($"Failed to complete channel for AWS request Id {awsRequestId}.");
+        }
+
         context.DurationTimer!.Stop();
 
         Logger?.LogInformation(