fix: keep advanced session message writes atomic

[he-yufeng]

Summary

• keep AdvancedSQLiteSession message inserts and structure metadata inserts in one transaction
• roll back and surface the metadata error instead of reporting a false-success write
• add a regression test that confirms no base-table or structure rows remain after failure

Fixes #3348.

To verify

• python -m py_compile src\agents\extensions\memory\advanced_sqlite_session.py tests\extensions\memory\test_advanced_sqlite_session.py
• uv run ruff check src\agents\extensions\memory\advanced_sqlite_session.py tests\extensions\memory\test_advanced_sqlite_session.py
• uv run --extra sqlalchemy pytest tests\extensions\memory\test_advanced_sqlite_session.py -q
• git diff --check