fix: #3348 AdvancedSQLiteSession.add_items silent partial save

[john-rocky]

Summary

AdvancedSQLiteSession.add_items first inserted the base message rows, committed them, and only then wrote message_structure rows. If the structure write failed, the matching conn.rollback() was a no-op because the base rows were already committed. A best-effort orphan cleanup ran next, but the original exception was always swallowed by self._logger.error(...), so the caller saw a successful add_items() even though:

• get_items() (which joins through message_structure) returned the rows as missing, and
• when the cleanup itself failed, the base table kept invisible orphan rows that no advanced read could ever surface.

The fix is to share a single transaction for both writes and let the original exception propagate. _insert_structure_metadata already reads back the just-inserted message IDs through a SELECT, which SQLite serves from the uncommitted state of the same transaction, so no intermediate commit is needed for correctness — only the false success was load-bearing.

Test plan

Added test_add_items_rolls_back_when_structure_metadata_fails. It subclasses AdvancedSQLiteSession with a toggleable _insert_structure_metadata that fails the first time and succeeds on retry, and verifies that:

• the failing call raises RuntimeError,
• both get_items() and direct counts on <messages_table> and message_structure come back empty,
• a follow-up call with the failure cleared persists the message normally.

$ uv run pytest tests/extensions/memory/test_advanced_sqlite_session.py
============================== 37 passed in 0.19s ==============================
$ make format
All checks passed!
$ make lint
All checks passed!
$ uv run mypy src/agents/extensions/memory/advanced_sqlite_session.py tests/extensions/memory/test_advanced_sqlite_session.py
Success: no issues found in 2 source files

Issue number

Closes #3348. Related to #3346 (orphan rows after delete_branch), which is a separate path and not touched here.

Checks

• I've added new tests (if relevant)
• I've added/updated the relevant documentation
• I've run `make lint` and `make format`
• I've made sure tests pass