Fix #57232 potential CancellationTokenSource leak in HubConnection (#58303)

Author: AshkanAfsharpour

src/SignalR/clients/csharp/Client.Core/src/HubConnection.cs

@@ -1596,7 +1596,7 @@ private async Task ReceiveLoop(ConnectionState connectionState)
         var timer = new TimerAwaitable(TickRate, TickRate);
         var timerTask = connectionState.TimerLoop(timer);
 
-        var uploadStreamSource = new CancellationTokenSource();
+        using var uploadStreamSource = new CancellationTokenSource();
         connectionState.UploadStreamToken = uploadStreamSource.Token;
         var invocationMessageChannel = Channel.CreateUnbounded<InvocationMessage>(_receiveLoopOptions);