PYTHON-5167 Properly cleanup test SocketGetter tasks

[ShaneHarvey]

PYTHON-5167 Properly cleanup test SocketGetter tasks

Should resolve one of the errors seen in SocketGetter.run_mongo_thread:

FAILED test/asynchronous/test_session.py::TestSession::test_end_sessions - pytest.PytestUnraisableExceptionWarning: Exception ignored in: <coroutine object SocketGetter.run_mongo_thread at 0x4dc35c00a00>

Traceback (most recent call last):
  File "/home/runner/work/mongo-python-driver/mongo-python-driver/pymongo/asynchronous/pool.py", line 1474, in _get_conn
    if not await _async_cond_wait(self.size_cond, timeout):
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
GeneratorExit

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/home/runner/work/mongo-python-driver/mongo-python-driver/test/asynchronous/test_pooling.py", line 120, in run_mongo_thread
    async with self.pool.checkout() as sock:
               ~~~~~~~~~~~~~~~~~~^^
  File "/home/runner/.local/share/uv/python/cpython-3.13.2+freethreaded-linux-x86_64-gnu/lib/python3.13t/contextlib.py", line 214, in __aenter__
    return await anext(self.gen)
           ^^^^^^^^^^^^^^^^^^^^^
  File "/home/runner/work/mongo-python-driver/mongo-python-driver/pymongo/asynchronous/pool.py", line 1355, in checkout
    conn = await self._get_conn(checkout_started_time, handler=handler)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/runner/work/mongo-python-driver/mongo-python-driver/pymongo/asynchronous/pool.py", line 1470, in _get_conn
    async with self.size_cond:
               ^^^^^^^^^^^^^^
  File "/home/runner/.local/share/uv/python/cpython-3.13.2+freethreaded-linux-x86_64-gnu/lib/python3.13t/asyncio/locks.py", line 20, in __aexit__
    self.release()
    ~~~~~~~~~~~~^^
  File "/home/runner/.local/share/uv/python/cpython-3.13.2+freethreaded-linux-x86_64-gnu/lib/python3.13t/asyncio/locks.py", line 142, in release
Error:     raise RuntimeError('Lock is not acquired.')
RuntimeError: Lock is not acquired.