Fix DashboardClient flaky test

[JamesNK]

Dispose could happen after we've check for dispose but before getting the token from the CTS.

Fix by taking a copy of the token when the CTS is created.

Test error:

System.ObjectDisposedException : The CancellationTokenSource has been disposed.
Stack trace
at System.Threading.CancellationTokenSource.get_Token() at Aspire.Dashboard.Model.DashboardClient.b__23_0() in //src/Aspire.Dashboard/Model/DashboardClient.cs:line 145 at System.Threading.Tasks.Task`1.InnerInvoke() at System.Threading.ExecutionContext.RunFromThreadPoolDispatchLoop(Thread threadPoolThread, ExecutionContext executionContext, ContextCallback callback, Object state) --- End of stack trace from previous location --- at System.Threading.ExecutionContext.RunFromThreadPoolDispatchLoop(Thread threadPoolThread, ExecutionContext executionContext, ContextCallback callback, Object state) at System.Threading.Tasks.Task.ExecuteWithThreadLocal(Task& currentTaskSlot, Thread threadPoolThread) --- End of stack trace from previous location --- at Aspire.Dashboard.Utils.TaskHelpers.WaitIgnoreCancelCoreAsync(Task task, ILogger logger, String logMessage) in //src/Shared/TaskHelpers.cs:line 29 at Aspire.Dashboard.Model.DashboardClient.DisposeAsync() in //src/Aspire.Dashboard/Model/DashboardClient.cs:line 389 at Aspire.Dashboard.Tests.Model.DashboardClientTests.SubscribeResources_OnDispose_ChannelRemoved() in //tests/Aspire.Dashboard.Tests/Model/DashboardClientTests.cs:line 69 --- End of stack trace from previous location ---

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