fix: resolve Jest integration test failures

- Remove duplicate mock file causing Jest warnings
- Add missing urlFor method to SeleniumHelper class
- Fix 'urlFor is not a function' errors in Ruby tab tests

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

Co-Authored-By: Claude <noreply@anthropic.com>

Author: takaokouji

src/lib/themes/dark/__mocks__/index.js

@@ -408,6 +408,15 @@ class SeleniumHelper {
             throw await enhanceError(outerError, cause);
         }
     }
+
+    urlFor (path) {
+        switch (path) {
+        case '/':
+            return pathModule.resolve(__dirname, '../../build/index.html');
+        default:
+            throw new Error(`Invalid path: ${path}`);
+        }
+    }
 }
 
 export default SeleniumHelper;