[Bug] Investigate `ExecuteNexusOperationAsync_SyncTimeout_FailsAsExpected` flake

[jmaeagle99]

Test output:

  Failed Temporalio.Tests.Worker.NexusWorkerTests.ExecuteNexusOperationAsync_SyncTimeout_FailsAsExpected [2 s]
  Error Message:
   System.ObjectDisposedException : The CancellationTokenSource has been disposed.
  Stack Trace:
     at System.Threading.CancellationTokenSource.get_WaitHandle()
   at Temporalio.Tests.Worker.NexusWorkerTests.<>c__DisplayClass12_0.<ExecuteNexusOperationAsync_SyncTimeout_FailsAsExpected>b__5() in /Users/runner/work/sdk-dotnet/sdk-dotnet/tests/Temporalio.Tests/Worker/NexusWorkerTests.cs:line 304
   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 Temporalio.Tests.Worker.NexusWorkerTests.ExecuteNexusOperationAsync_SyncTimeout_FailsAsExpected() in /Users/runner/work/sdk-dotnet/sdk-dotnet/tests/Temporalio.Tests/Worker/NexusWorkerTests.cs:line 304
--- End of stack trace from previous location ---