refactor(models): Refine MessageAgentThought SQLAlchemy typing

[laipz8200]

Summary

• Fixes [Chore/Refactor] Tighten MessageAgentThought type annotations #27748.
• Add explicit Mapped[...] annotations to MessageAgentThought text, numeric, and temporal columns for SQLAlchemy 2.0 typing.
• Ensure pricing fields rely on Decimal typing so static analysis can treat message cost calculations consistently.

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 significantly enhances the type safety and clarity of the MessageAgentThought SQLAlchemy model. By introducing explicit Mapped[...] annotations for various column types and standardizing pricing fields to use Decimal, the changes improve code maintainability, facilitate better static analysis, and ensure more reliable data handling within the application's core models.

Highlights

• SQLAlchemy Typing Refinement: Explicit Mapped[...] annotations have been added to various text, numeric, and temporal columns within the MessageAgentThought model to align with SQLAlchemy 2.0 typing best practices.
• Pricing Field Consistency: Pricing-related fields (message_unit_price, message_price_unit, answer_unit_price, answer_price_unit, total_price) now explicitly use Decimal typing, ensuring consistent static analysis for message cost calculations.
• Issue Resolution: This pull request addresses and fixes issue [Chore/Refactor] Tighten MessageAgentThought type annotations #27748, improving the robustness and type safety of the MessageAgentThought model.

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[gemini-code-assist]

Code Review

This pull request refines the SQLAlchemy typing for the MessageAgentThought model, aligning it with modern SQLAlchemy 2.0 practices. The changes correctly add Mapped annotations and use Decimal for pricing fields, which improves type safety and consistency for static analysis. I have one suggestion to further improve type safety for the created_by_role field by using the CreatorUserRole enum.