Guard WaitForExit() with HasExited check to avoid hanging if kill times out

If KillTree's kill succeeds but WaitForExit(int) times out, the process
is still alive. Calling WaitForExit() (no args) unconditionally would
then block indefinitely. Guard with HasExited to only flush stderr
events when we know the process has actually exited.

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

Author: Copilot

src/ModelContextProtocol.Core/Client/StdioClientTransport.cs

@@ -230,8 +230,12 @@ internal static void DisposeProcess(
 
                 // Ensure all redirected stderr/stdout events have been dispatched
                 // before disposing. Only the no-arg WaitForExit() guarantees this;
-                // WaitForExit(int) (used by KillTree) does not.
-                process.WaitForExit();
+                // WaitForExit(int) (used by KillTree) does not. Guard with HasExited
+                // to avoid hanging indefinitely if the process couldn't be killed.
+                if (HasExited(process))
+                {
+                    process.WaitForExit();
+                }
             }
             finally
             {