refactor(models): Add mapped type hints to MessageAnnotation

[laipz8200]

Summary

• Fixes [Chore/Refactor] Add SQLAlchemy mapped type hints to MessageAnnotation #27750
• Add Mapped type hints on message annotation fields to align with typed SQLAlchemy models
• Clarify timestamp column typing with explicit datetime annotations

Screenshots

Before	After
N/A	N/A

Checklist

• 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

[gemini-code-assist]

Summary of Changes

Hello @laipz8200, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request enhances the 'MessageAnnotation' SQLAlchemy model by introducing 'Mapped' type hints for various fields and explicitly annotating timestamp columns with 'datetime'. These changes improve type consistency and clarity, aligning the model with modern SQLAlchemy typing practices and resolving a reported issue.

Highlights

• Type Hinting Consistency: Introduced 'Mapped' type hints for several fields within the 'MessageAnnotation' model to align with typed SQLAlchemy conventions.
• Timestamp Clarity: Explicitly annotated 'created_at' and 'updated_at' columns with 'datetime' type hints for improved clarity.
• Issue Resolution: This pull request addresses and fixes issue [Chore/Refactor] Add SQLAlchemy mapped type hints to MessageAnnotation #27750.

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[gemini-code-assist]

Code Review

This pull request adds Mapped type hints to several fields in the MessageAnnotation model in api/models/model.py. Specifically, question, content, account_id, created_at, and updated_at are now explicitly typed. This is a good improvement that aligns with modern SQLAlchemy practices, enhancing code clarity and enabling better static type checking. The changes are correct and well-implemented. No issues found.

[laipz8200]

It seems message_annotations.question has never been None; otherwise, it would mean we’ve had a bug here for a long time.

cc @snakevash