tests(config): add subTest per model and cover list branch in handler tests

- Wrap each model_name iteration in self.subTest(model_name=model_name) so
  failures in TestHandlersTransaction clearly report which model caused the issue
- Add test_devicegroup_templates_change_handler_list_branch to cover the
  list branch of devicegroup_templates_change_handler (type(instance) is list),
  verifying change_devices_templates.delay fires after commit and not on rollback

Author: prakash-kalwaniya

openwisp_controller/config/tests/test_handlers_transaction.py

@@ -41,32 +41,65 @@ def __init__(self, id, model_name):
         ]
 
         for model_name in model_names:
-            mock_instance = MockInstance(id="test-id", model_name=model_name)
-            kwargs = {}
-            if model_name == Config._meta.model_name:
-                kwargs["backend"] = "test-backend"
-                kwargs["old_backend"] = "old-backend"
-            elif model_name == DeviceGroup._meta.model_name:
-                kwargs["templates"] = []
-                kwargs["old_templates"] = []
-            elif model_name == Device._meta.model_name:
-                kwargs["group_id"] = "test-group"
-                kwargs["old_group_id"] = "old-group"
-
-            # Case 1: Transaction commits
+            with self.subTest(model_name=model_name):
+                mock_instance = MockInstance(id="test-id", model_name=model_name)
+                kwargs = {}
+                if model_name == Config._meta.model_name:
+                    kwargs["backend"] = "test-backend"
+                    kwargs["old_backend"] = "old-backend"
+                elif model_name == DeviceGroup._meta.model_name:
+                    kwargs["templates"] = []
+                    kwargs["old_templates"] = []
+                elif model_name == Device._meta.model_name:
+                    kwargs["group_id"] = "test-group"
+                    kwargs["old_group_id"] = "old-group"
+
+                # Case 1: Transaction commits
+                mock_delay.reset_mock()
+                with transaction.atomic():
+                    devicegroup_templates_change_handler(mock_instance, **kwargs)
+                    # Should not be called inside the transaction
+                    mock_delay.assert_not_called()
+
+                mock_delay.assert_called_once()  # Should be called after commit
+
+                # Case 2: Transaction rolls back
+                mock_delay.reset_mock()
+                try:
+                    with transaction.atomic():
+                        devicegroup_templates_change_handler(mock_instance, **kwargs)
+                        mock_delay.assert_not_called()
+                        raise IntegrityError("Force rollback")
+                except IntegrityError:
+                    pass
+
+                # Should not be called because it rolled back
+                mock_delay.assert_not_called()
+
+    @patch("openwisp_controller.config.tasks.change_devices_templates.delay")
+    def test_devicegroup_templates_change_handler_list_branch(self, mock_delay):
+        """
+        Test that the list branch of devicegroup_templates_change_handler
+        (type(instance) is list) correctly dispatches change_devices_templates.delay
+        after commit and not on rollback.
+        """
+        instance_list = ["device-id-1", "device-id-2"]
+        kwargs = {"group_id": "test-group", "old_group_id": "old-group"}
+
+        with self.subTest("list branch: delay called after commit"):
             mock_delay.reset_mock()
             with transaction.atomic():
-                devicegroup_templates_change_handler(mock_instance, **kwargs)
+                devicegroup_templates_change_handler(instance_list, **kwargs)
                 # Should not be called inside the transaction
                 mock_delay.assert_not_called()
 
             mock_delay.assert_called_once()  # Should be called after commit
 
-            # Case 2: Transaction rolls back
+        with self.subTest("list branch: delay not called on rollback"):
             mock_delay.reset_mock()
             try:
                 with transaction.atomic():
-                    devicegroup_templates_change_handler(mock_instance, **kwargs)
+                    devicegroup_templates_change_handler(instance_list, **kwargs)
                     mock_delay.assert_not_called()
                     raise IntegrityError("Force rollback")
             except IntegrityError: