Fix race condition in DisposeProcess that could skip flushing stderr events

When GetUnexpectedExitExceptionAsync sees HasExited as false and returns
null, DisposeProcess is called next. By that time the process may have
exited, causing it to skip KillTree and call Dispose() without first
calling WaitForExit() to flush ErrorDataReceived callbacks. Add a
WaitForExit() call for already-exited processes to ensure all redirected
output events are dispatched before disposal.

Co-authored-by: stephentoub <2642209+stephentoub@users.noreply.github.com>

Author: Copilot

src/ModelContextProtocol.Core/Client/StdioClientTransport.cs

@@ -227,6 +227,13 @@ internal static void DisposeProcess(
                     // and Node.js does not kill its children when it exits properly.
                     process.KillTree(shutdownTimeout);
                 }
+                else if (HasExited(process))
+                {
+                    // The process has already exited. Call WaitForExit() (no arguments)
+                    // to ensure all redirected stderr/stdout events have been dispatched
+                    // before disposing.
+                    process.WaitForExit();
+                }
             }
             finally
             {