docs(mcp): prefer -d web-server for agent-driven web testing

With `-d chrome`, only the Chrome window Flutter spawns receives hot
reload patches; a second browser at the same URL renders the bundle but
is invisible to dwds. Spell out the recommendation in the note block so
readers don't hit the trap.

Author: Ortes

src/content/ai/mcp-server.md

@@ -415,7 +415,9 @@ out of production builds.
 **Web**: the `flutter_driver` package doesn't compile under dart2js. Pair
 the Dart MCP server with a browser-driving MCP for clicks and screenshots;
 the Dart MCP server still handles widget tree, runtime errors, and hot
-reload through DTD.
+reload through DTD. Prefer `flutter run -d web-server` so the browser the
+agent drives is the one DTD is connected to — with `-d chrome`, only the
+window Flutter spawned receives hot reload patches.
 
 See the [setup guide][flutter-driver-guide] for the snippet, web modes,
 and common pitfalls.