fix(graphql): resolve duplicate events on socket reconnect

Resolved an issue in WebSocketClient where multiple listeners were added to `waitForConnectedState` when socket reconnects after subscription been requtesed with disconnected socket, causing duplicate events to be received by subscriptions.

packages/graphql/lib/src/links/websocket_link/websocket_client.dart

@@ -502,6 +502,7 @@ class SocketClient {
             )
           : waitForConnectedStateWithoutTimeout;
 
+      sub?.cancel();
       sub = waitForConnectedState.listen((_) {
         final Stream<GraphQLSocketMessage> dataErrorComplete = _messages.where(
           (GraphQLSocketMessage message) {