Fix StdioServerTransport.DisposeAsync() hang: CancellableStdinStream missing Dispose override (#1276)

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: stephentoub <2642209+stephentoub@users.noreply.github.com>

src/ModelContextProtocol.Core/Server/StdioServerTransport.cs

@@ -59,6 +59,16 @@ public sealed class StdioServerTransport : StreamServerTransport
         }
 #endif
 
+        protected override void Dispose(bool disposing)
+        {
+            if (disposing)
+            {
+                stdinStream.Dispose();
+            }
+
+            base.Dispose(disposing);
+        }
+
         // The McpServer shouldn't call flush on the stdin Stream, but it doesn't need to throw just in case.
         public override void Flush() { }