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

[edgarrmondragon]

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.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.01%. Comparing base (6deaf3b) to head (33fae08).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1017      +/-   ##
==========================================
+ Coverage   82.82%   83.01%   +0.19%     
==========================================
  Files          16       16              
  Lines        1723     1725       +2     
  Branches      189      189              
==========================================
+ Hits         1427     1432       +5     
+ Misses        240      236       -4     
- Partials       56       57       +1     

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