refactor(api): use sessionmaker in relyt & tidb_vector VDB services

[carlos4s]

Summary

• Replace with Session(self.client/self._engine) + session.commit() with
  sessionmaker(bind=...).begin() in relyt (2 blocks) and tidb_vector (2 blocks)
• Read-only session blocks left unchanged
• Updated test mocks to match new sessionmaker pattern

Part of #24245

Test plan

• ruff check passes
• 42 relyt + tidb_vector tests pass

[github-actions]

Pyrefly Diff

No changes detected.

[Copilot]

Pull request overview

Refactors write-transaction handling in the Relyt and TiDB Vector DB services to use sessionmaker(bind=...).begin() instead of manually calling session.begin()/session.commit(), and updates unit tests accordingly.

Changes:

• Switched RelytVector write operations (create_collection, delete) to sessionmaker(...).begin() context manager transactions.
• Switched TiDBVector write operations (_create_collection, delete) to sessionmaker(...).begin() context manager transactions.
• Updated unit test monkeypatch/mocks to align with the new sessionmaker-based transaction pattern.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
api/core/rag/datasource/vdb/tidb_vector/tidb_vector.py	Uses sessionmaker(...).begin() for TiDB write transactions (create/drop table).
api/core/rag/datasource/vdb/relyt/relyt_vector.py	Uses sessionmaker(...).begin() for Relyt write transactions (create/drop table).
api/tests/unit_tests/core/rag/datasource/vdb/tidb_vector/test_tidb_vector.py	Updates mocks/patching for TiDB sessionmaker .begin() context manager.
api/tests/unit_tests/core/rag/datasource/vdb/relyt/test_relyt_vector.py	Adds shared patch helper for Session+sessionmaker; updates tests for new transaction semantics.

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[carlos4s]

@asukaminato0721 @crazywoola
Thanks for merging this.