refactor: migrate MessageAnnotation to TypeBase

[sxxtony]

Summary

• Migrate MessageAnnotation from Base to TypeBase (MappedAsDataclass)
• Add Mapped type annotations and dataclass-compatible mapped_column parameters
• Reorder fields: required fields before optional ones to satisfy dataclass rules
• Auto-generated fields (id, created_at, updated_at) use init=False

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 make lint and make type-check (backend) and cd web && npx lint-staged (frontend) to appease the lint gods

Part of #23579

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[Copilot]

Pull request overview

Refactors the MessageAnnotation ORM model to use TypeBase (SQLAlchemy MappedAsDataclass) and updates its mapped fields to be dataclass-compatible.

Changes:

• Migrated MessageAnnotation from Base to TypeBase.
• Updated mapped columns to use Mapped[...] annotations and dataclass-friendly defaults (default_factory, init=False, and required-before-optional ordering).
• Adjusted defaults for nullable fields and counters (conversation_id, message_id, hit_count) and updated timestamp column configuration.

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