Fix race in KestrelInMemoryConnection

[halter73]

This should avoid the following race. I went the easy route, and just stop disposing KestrelInMemoryConnection._connectionClosedCts, because IIRC, you only really need to dispose it if a caller is using something like CancellationToken.WaitHandle, which I don't expect in our tests at least. I could try synchronizing with the SocketsHttpHandler disposing the stream on a background thread, but it was a lot more code and risked deadlocks if done incorrectly.

[xUnit.net 00:00:06.33]       System.AggregateException : One or more errors occurred. (The CancellationTokenSource has been disposed.)
[xUnit.net 00:00:06.33]     [FATAL ERROR] System.AggregateException
[xUnit.net 00:00:06.33]       ---- System.ObjectDisposedException : The CancellationTokenSource has been disposed.
[xUnit.net 00:00:06.33]       Stack Trace:
[xUnit.net 00:00:06.33]            at System.Threading.CancellationTokenSource.ExecuteCallbackHandlers(Boolean throwOnFirstException)
[xUnit.net 00:00:06.33]            at System.Threading.TimerQueueTimer.Fire(Boolean isThreadPool)
[xUnit.net 00:00:06.33]            at System.Threading.TimerQueue.FireNextTimers()
[xUnit.net 00:00:06.33]            at System.Threading.ThreadPoolWorkQueue.Dispatch()
[xUnit.net 00:00:06.33] Catastrophic failure: System.AggregateException : One or more errors occurred. (The CancellationTokenSource has been disposed.)
[xUnit.net 00:00:06.33]            at System.Threading.PortableThreadPool.WorkerThread.WorkerThreadStart()
---- System.ObjectDisposedException : The CancellationTokenSource has been disposed.
[xUnit.net 00:00:06.33]         ----- Inner Stack Trace -----
[xUnit.net 00:00:06.33]            at System.Threading.CancellationTokenSource.Cancel()
[xUnit.net 00:00:06.33]         /_/tests/ModelContextProtocol.Tests/Utils/KestrelInMemoryConnection.cs(83,0): at ModelContextProtocol.Tests.Utils.KestrelInMemoryConnection.DuplexStream.Dispose(Boolean disposing)
[xUnit.net 00:00:06.33]            at System.IO.Stream.Close()
[xUnit.net 00:00:06.33]            at System.Threading.CancellationTokenSource.Invoke(Delegate d, Object state, CancellationTokenSource source)
[xUnit.net 00:00:06.33]            at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state)
[xUnit.net 00:00:06.33]         --- End of stack trace from previous location ---
[xUnit.net 00:00:06.33]            at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state)
[xUnit.net 00:00:06.33]            at System.Threading.CancellationTokenSource.ExecuteCallbackHandlers(Boolean throwOnFirstException)

https://github.com/modelcontextprotocol/csharp-sdk/actions/runs/14319353286/job/40132789327

• Also, this removes some duplication from WithStdioServerTransport and WithStreamServerTransport