Replace five # type: ignore comments with setattr() calls (for module
attribute monkey-patching) and an Any return type (for _get_memory_engines),
eliminating all type suppressions per project zero-tolerance policy.
Fix ruff format violations: collapse @when decorator to one line, reformat
raise...from split, and reformat assert message wrapping.
ISSUES CLOSED: #7566
Add MEMORY_ENGINES_LOCK to engine_cache.py and wrap the check-and-set
in UnitOfWork.engine with it to prevent concurrent threads from creating
duplicate in-memory SQLite engines. Also fix cache-hit bug where
self._engine was never assigned on a cache hit.
Closes#7566
HAL9000