refactor(api): inject sessionmaker into conversation variable updater

[laipz8200]

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Summary

Follow-up #30531 (comment)

• Inject sessionmaker into ConversationVariableUpdater and remove the factory indirection.
• Update callers to build the updater with the shared session factory instead of accessing db.engine.
• Motivation: avoid per-update engine access and improve dependency injection consistency.
• Dependencies: none.

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 refactors the ConversationVariableUpdater class to enhance its modularity and testability. The updater now receives a SQLAlchemy sessionmaker via its constructor, eliminating its direct dependency on a global database engine. This change promotes better dependency injection practices across the application where the updater is used.

Highlights

• Dependency Injection: The ConversationVariableUpdater now accepts a sessionmaker during initialization, moving away from direct global db.engine usage.
• Improved Testability: By injecting the sessionmaker, the ConversationVariableUpdater becomes easier to test in isolation.
• Code Cleanup: The conversation_variable_updater_factory function has been removed, simplifying the instantiation process.

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[gemini-code-assist]

Code Review

This pull request refactors the ConversationVariableUpdater by renaming ConversationVariableUpdaterImpl to ConversationVariableUpdater and modifying its constructor to accept a sessionmaker for database session management, thereby decoupling it from a global db.engine instance. The conversation_variable_updater_factory function has been removed, and ConversationVariableUpdater is now instantiated directly with session_factory.get_session_maker() in app_runner.py and conversation_service.py. Corresponding import statements were updated across the affected files.