PYTHON-5672 Fix MockPool.checkout() bypass in _ClientCheckout

Constructing _PoolCheckout directly bypassed pool.checkout() overrides
used in MockPool, causing mock_down_hosts checks to be skipped and
network-error tests to fail.

Author: blink1073

pymongo/asynchronous/mongo_client.py

@@ -2716,7 +2716,7 @@ async def __aenter__(self) -> AsyncConnection:
         if in_txn and session and session._pinned_connection:
             self.contribute_socket(session._pinned_connection)
             return session._pinned_connection
-        pool_checkout = _PoolCheckout(server.pool, self)
+        pool_checkout = server.pool.checkout(self)
         try:
             conn = await pool_checkout.__aenter__()
         except BaseException as exc:

pymongo/synchronous/mongo_client.py

@@ -2705,7 +2705,7 @@ def __enter__(self) -> Connection:
         if in_txn and session and session._pinned_connection:
             self.contribute_socket(session._pinned_connection)
             return session._pinned_connection
-        pool_checkout = _PoolCheckout(server.pool, self)
+        pool_checkout = server.pool.checkout(self)
         try:
             conn = pool_checkout.__enter__()
         except BaseException as exc: