fix: inputs variables value changed but not work on Chatflow App

[liuzhenghua]

Summary

This PR enables the Chatflow app to change the values of variables in inputs with each message sent.

Tip

Close issue syntax: Fixes #<issue number> or Resolves #<issue number>, see documentation for more details.

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Checklist

Important

Please review the checklist below before submitting your pull request.

• This change requires a documentation update, included: Dify Document
• I understand that this PR may be closed in case there was no previous discussion or issues. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.
• I ran dev/reformat(backend) and cd web && npx lint-staged(frontend) to appease the lint gods

[liuzhenghua]

you can use a simple workflow which contains a code node to test it.

[crazywoola]

Please see this as well #10652

[liuzhenghua]

Please see this as well #10652

I think it's not very reasonable to directly read the user's inputs every time. For example, some variables are only set at the beginning of the conversation, and there’s no need to pass them around later. So, my code checks if the inputs are empty, and if so, it continues to use the session's inputs.

[liuzhenghua]

I think modifying variables only makes sense in the Chatflow App. For a simple chat assistant, variables can only be placed in the prompt, specifically in system-type messages. Once the conversation has started, changing them doesn't have much significance.