Add two new test classes to verify:
- ConnectionPool does not leak connections (fresh conn after close,
context manager cleanup, thread isolation, repeated open/close cycles)
- SQLite pragmas (WAL, busy_timeout, cache_size) are correctly applied
and scoped per-connection as used throughout the codebase
Fixes#944
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>