unit test fixes + error refactor

src/routes/user.routes.test.ts

@@ -283,7 +283,7 @@ describe('User Routes (/api/users)', () => {
       it('should return 400 for an invalid listId on DELETE', async () => {
         const response = await supertest(app).delete('/api/users/shopping-lists/abc');
         expect(response.status).toBe(400);
-        expect(response.body.message).toBe('Invalid list ID.');
+        expect(response.body.message).toBe("Invalid ID for parameter 'listId'. Must be a number.");
       });
 
       describe('DELETE /shopping-lists/:listId', () => {