fix(promptMessage): correct field_serializer implementation for content serialization

[Nov1c444]

Summary

Added a field_serializer for the "content" attribute in UserPromptMessage to properly handle different content types:

• Return content as-is for None or string values
• Process list-type content by converting model objects to dictionaries
• Return other content types unchanged

This fixes serialization issues when UserPromptMessage contains complex content structures, especially when lists include model objects that need to be properly serialized for JSON conversion.

Fixes #18355

Screenshots

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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