fix(threejs-server): use server.registerTool for non-UI tool

The learn_threejs tool doesn't have a UI component - it only returns
documentation text. Using registerAppTool without _meta causes a
runtime error when the function tries to access config._meta.ui.

Fixes crash: "Cannot read property 'ui' of undefined"

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: ochafik

examples/threejs-server/server.ts

@@ -173,9 +173,8 @@ function createServer(): McpServer {
     },
   );
 
-  // Tool 2: learn_threejs
-  registerAppTool(
-    server,
+  // Tool 2: learn_threejs (not a UI tool, just returns documentation)
+  server.registerTool(
     "learn_threejs",
     {
       title: "Learn Three.js",