GeezerVison

• Issue: sqlite3 converter lookup can crash during concurrent registry updates in free-threaded builds #155781

Use PyDict_GetItemRef() when looking up registered SQLite converters so the converter remains alive until it has been added to the cursor's row cast map.

Update the caller's reference ownership and add a free-threading regression test that concurrently builds row cast maps and modifies the public converter registry.

Tests:

• ./python -m test -v test_free_threading.test_sqlite3
• ./python -m test -v test_sqlite3
• make patchcheck