Add timeout to WaitForExitAsync in GetUnexpectedExitExceptionAsync

When CleanupAsync is entered from ReadMessagesAsync (stdout EOF), the
cancellation token is _shutdownCts.Token which hasn't been canceled yet
(base.CleanupAsync runs later). If stderr EOF hasn't been delivered by
the threadpool yet, WaitForExitAsync hangs indefinitely.

Use a linked CTS with ShutdownTimeout to bound the wait. The process is
already dead; we're just draining stderr pipe buffers.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: ericstj

src/ModelContextProtocol.Core/Client/StdioClientSessionTransport.cs

@@ -99,8 +99,14 @@ protected override async ValueTask CleanupAsync(Exception? error = null, Cancell
         try
         {
             // The process has exited, but we still need to ensure stderr has been flushed.
+            // Use a bounded wait: the process is already dead, we're just draining pipe
+            // buffers. If the caller's token is never canceled (e.g. _shutdownCts hasn't
+            // been canceled yet), an unbounded wait here can hang indefinitely when the
+            // threadpool is slow to deliver the stderr EOF callback.
 #if NET
-            await _process.WaitForExitAsync(cancellationToken).ConfigureAwait(false);
+            using var timeoutCts = CancellationTokenSource.CreateLinkedTokenSource(cancellationToken);
+            timeoutCts.CancelAfter(_options.ShutdownTimeout);
+            await _process.WaitForExitAsync(timeoutCts.Token).ConfigureAwait(false);
 #else
             _process.WaitForExit((int)_options.ShutdownTimeout.TotalMilliseconds);
 #endif