PYTHON-4414 Fix order, add then discard is safer

Author: ShaneHarvey

pymongo/asynchronous/pool.py

@@ -1296,8 +1296,8 @@ async def connect(self, handler: Optional[_MongoClientErrorHandler] = None) -> A
 
         conn = AsyncConnection(sock, self, self.address, conn_id)  # type: ignore[arg-type]
         async with self.lock:
-            self.active_contexts.discard(tmp_context)
             self.active_contexts.add(conn.cancel_context)
+            self.active_contexts.discard(tmp_context)
         if tmp_context.cancelled:
             conn.cancel_context.cancel()
         try:

pymongo/synchronous/pool.py

@@ -1290,8 +1290,8 @@ def connect(self, handler: Optional[_MongoClientErrorHandler] = None) -> Connect
 
         conn = Connection(sock, self, self.address, conn_id)  # type: ignore[arg-type]
         with self.lock:
-            self.active_contexts.discard(tmp_context)
             self.active_contexts.add(conn.cancel_context)
+            self.active_contexts.discard(tmp_context)
         if tmp_context.cancelled:
             conn.cancel_context.cancel()
         try: