docs: clarify Copilot instructions for npm test

.github/copilot-instructions.md

@@ -55,6 +55,7 @@
   - Always run after code changes to backend logic.
   - If `vendor/bin/phpunit` does not exist, ensure `composer install` completed without errors.
 
+- **PHP JS Tests:**
   ```sh
   cd ./js
   npm run test