refactor: migrate session.query to select API in document indexing sync task

[RenzoMXD]

Summary

• Migrate session.query() to SQLAlchemy 2.0 select() API in tasks/document_indexing_sync_task.py
• 7× .query(Model).where(...).first() / .filter_by(...).first() → session.scalar(select(Model).where(...).limit(1))
• Minimal test mock updates: .query.return_value.where.return_value.first.side_effect → session.scalar.side_effect

Test plan

• All 4 unit tests pass
• basedpyright: 0 errors, 0 warnings
• mypy: 0 errors

Part of #22668

[github-actions]

Pyrefly Diff

No changes detected.

[Copilot]

Pull request overview

Refactors the Notion document indexing sync Celery task to use SQLAlchemy 2.0 select()-style ORM reads, improving typing and modernizing query usage in line with issue #22668.

Changes:

• Replaced multiple session.query(...).where/filter_by(...).first() lookups with session.scalar(select(...).where(...).limit(1)) in document_indexing_sync_task.
• Updated unit test DB session mocks to reflect the new session.scalar() call pattern.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
api/tasks/document_indexing_sync_task.py	Migrates ORM reads from legacy Query API to SQLAlchemy 2.0 select() + session.scalar() while preserving existing control flow.
api/tests/unit_tests/tasks/test_document_indexing_sync_task.py	Adjusts session mocks from chained .query().where().first() to session.scalar.side_effect to match the refactor.

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