Fix OperationCount calculation (#3095)

Parenthesize `??` as `+` binds stronger

Author: jnyrup

src/StackExchange.Redis/ServerEndPoint.cs

@@ -180,7 +180,7 @@ internal Exception? LastException
         internal State InteractiveConnectionState => interactive?.ConnectionState ?? State.Disconnected;
         internal State SubscriptionConnectionState => KnowOrAssumeResp3() ? InteractiveConnectionState : subscription?.ConnectionState ?? State.Disconnected;
 
-        public long OperationCount => interactive?.OperationCount ?? 0 + subscription?.OperationCount ?? 0;
+        public long OperationCount => (interactive?.OperationCount ?? 0) + (subscription?.OperationCount ?? 0);
 
         public bool RequiresReadMode => serverType == ServerType.Cluster && IsReplica;