fix: #3348 propagate add_items structure metadata failure

[kimnamu]

Summary

Fixes #3348.

add_items() wraps _insert_structure_metadata() in a try/except that rolls back, logs, and runs orphan cleanup — but then returns normally, so callers can't tell a partial failure from a success.

This adds a single raise RuntimeError(...) from e after the existing cleanup block, so the failure surfaces to the caller while preserving the original exception via from e. The existing rollback, logging, and cleanup behaviour is untouched.

Before / After

Scenario	Before (main)	After (this PR)
_insert_structure_metadata() raises	Exception swallowed; add_items() returns normally	RuntimeError raised with original exception chained via from e
Caller observability	Cannot distinguish partial failure from success	Failure surfaces to the caller
Rollback / logging / orphan cleanup	Performed	Performed (unchanged)
Successful path	Returns normally	Returns normally (unchanged)

Test

Adds test_add_items_raises_on_structure_metadata_failure, which forces _insert_structure_metadata to raise, then asserts add_items() raises RuntimeError and that the orphaned message was still cleaned up.

$ uv run pytest tests/extensions/memory/test_advanced_sqlite_session.py::test_add_items_raises_on_structure_metadata_failure -v
============================= test session starts ==============================
tests/extensions/memory/test_advanced_sqlite_session.py::test_add_items_raises_on_structure_metadata_failure PASSED [100%]
============================== 1 passed in 0.18s ===============================

Note: this is one half of #3498, split into a single-issue PR as suggested for easier review. The other half (#3346) is in #3526.