chore(test): adapt mongoose test to mongoose v9

_id is ObjectId and cannot be cast to string.

Author: orgads

tests/e2e/mongoose.spec.ts

@@ -58,7 +58,7 @@ describe('Mongoose', () => {
       ...createDto,
       name: 'Nest',
       age: 5,
-      kitten: [kitten._id as string],
+      kitten: [kitten._id.toString()],
     };
 
     const parent = await new Promise<string>((resolve) => {