Only call WaitForExit() when process exited on its own, not after KillTree

The previous fix called WaitForExit() unconditionally or guarded only by
HasExited, which could hang on Windows when KillTree left child processes
holding output handles. Now WaitForExit() is only called when the process
exited before DisposeProcess was called (!processRunning), meaning KillTree
was not invoked. In that case, no child processes hold handles and
WaitForExit() completes quickly.

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

Author: Copilot

src/ModelContextProtocol.Core/Client/StdioClientTransport.cs

@@ -230,9 +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. Guard with HasExited
-                // to avoid hanging indefinitely if the process couldn't be killed.
-                if (HasExited(process))
+                // WaitForExit(int) (as used by KillTree) does not.
+                // This should not hang: either the process already exited on its own
+                // (no child processes holding handles), or KillTree killed the entire
+                // process tree. If it does take too long, the test infrastructure's
+                // own timeout will catch it.
+                if (!processRunning && HasExited(process))
                 {
                     process.WaitForExit();
                 }