Always call WaitForExit() (no args) before Dispose() in DisposeProcess

The previous fix only called WaitForExit() in the else branch (process
already exited). But when the process is still running, KillTree is
called which uses either Kill+WaitForExit(int) or returns early if the
process already exited - neither of which guarantees ErrorDataReceived
event dispatch. Move WaitForExit() (no args) to be unconditional after
the if/else block so it always runs before Dispose().

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

Author: Copilot

src/ModelContextProtocol.Core/Client/StdioClientTransport.cs

@@ -227,13 +227,11 @@ internal static void DisposeProcess(
                     // and Node.js does not kill its children when it exits properly.
                     process.KillTree(shutdownTimeout);
                 }
-                else
-                {
-                    // The process has already exited. Call WaitForExit() (no arguments)
-                    // to ensure all redirected stderr/stdout events have been dispatched
-                    // before disposing.
-                    process.WaitForExit();
-                }
+
+                // 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();
             }
             finally
             {