fix(core): close Reaper socket at interpreter exit to prevent ResourceWarning (#1017)

The Ryuk keep-alive socket stored in `Reaper._socket` was never
explicitly
closed, so CPython's GC finalized it during interpreter shutdown and
emitted
a ResourceWarning ("unclosed <socket.socket ...>") that is impossible to
suppress via warnings filters because it fires after pytest exits.

```
================================================ slowest 10 durations =================================================
12.25s setup    tests/test_integration.py::TestIntegration::test_ensure_tables_creates_schema_if_not_exists
0.26s call     tests/test_integration.py::TestIntegration::test_ensure_tables_creates_schema_if_not_exists
0.25s teardown tests/test_integration.py::TestIntegration::test_ensure_tables_creates_schema_if_not_exists
0.21s call     tests/test_state.py::test_acquire_lock_max_retries_exceeded

(6 durations < 0.005s hidden.  Use -vv to show these durations.)
=========================================== 23 passed, 1 warning in 13.04s ============================================
Exception ignored in: <socket.socket fd=22, family=AddressFamily.AF_INET, type=SocketKind.SOCK_STREAM, proto=0, laddr=('127.0.0.1', 55812)>
ResourceWarning: unclosed <socket.socket fd=22, family=AddressFamily.AF_INET, type=SocketKind.SOCK_STREAM, proto=0, laddr=('127.0.0.1', 55812)>
```

Registering `Reaper.delete_instance` with `atexit` ensures the socket is
closed
cleanly before the GC sweep, eliminating the warning without changing
the
socket's lifetime during the test session.

Signed-off-by: Edgar Ramírez Mondragón <edgarrm358@gmail.com>