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

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.

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>

Author: edgarrmondragon

core/testcontainers/core/container.py

@@ -1,3 +1,4 @@
+import atexit
 import contextlib
 import io
 import pathlib
@@ -476,5 +477,6 @@ def _create_instance(cls) -> "Reaper":
         rs.send(f"label={LABEL_SESSION_ID}={SESSION_ID}\r\n".encode())
 
         Reaper._instance = Reaper()
+        atexit.register(Reaper.delete_instance)
 
         return Reaper._instance