fix: resolve strapi instance reference in test helpers

Fix ReferenceError in cleanupStrapi() by using the local instance
variable instead of global strapi. Add null check to prevent cleanup
errors when instance is not initialized.

Author: MSACC

playground/__tests__/helpers.js

@@ -15,13 +15,17 @@ async function setupStrapi() {
 }
 
 async function cleanupStrapi() {
-  const dbSettings = strapi.config.get('database.connection');
+  if (!instance) {
+    return;
+  }
+
+  const dbSettings = instance.config.get('database.connection');
 
   // close server to release the db-file.
-  await strapi.server.httpServer.close();
+  await instance.server.httpServer.close();
 
   // close the connection to the database before deletion.
-  await strapi.db.connection.destroy();
+  await instance.db.connection.destroy();
 
   // delete test database after all tests have completed.
   if (dbSettings && dbSettings.connection && dbSettings.connection.filename) {