Return alert groups for deleted integrations on private api (#3223)

# What this PR does

This PR adds alert groups from deleted integrations to the api response.
More details in this [slack
thread](https://raintank-corp.slack.com/archives/C0229FD3CE9/p1698742008962209)

## Which issue(s) this PR fixes

## Checklist

- [ ] Unit, integration, and e2e (if applicable) tests updated
- [ ] Documentation added (or `pr:no public docs` PR label added if not
required)
- [ ] `CHANGELOG.md` updated (or `pr:no changelog` PR label added if not
required)

---------

Co-authored-by: Matvey Kukuy <Matvey-Kuk@users.noreply.github.com>

engine/apps/api/tests/test_alert_group.py

@@ -76,6 +76,27 @@ def test_get_filter_by_integration(
     assert len(response.data["results"]) == 4
 
 
+@pytest.mark.django_db
+def test_get_alert_groups_from_deleted_integration(alert_group_internal_api_setup, make_user_auth_headers):
+    user, token, alert_groups = alert_group_internal_api_setup
+
+    alert_receive_channel = alert_groups[0].channel
+    alert_receive_channel.delete()
+
+    client = APIClient()
+
+    url = reverse("api-internal:alertgroup-list")
+    response = client.get(
+        url,
+        format="json",
+        **make_user_auth_headers(user, token),
+    )
+
+    assert response.status_code == status.HTTP_200_OK
+    # Alert groups from deleted integrations should be returned
+    assert len(response.data["results"]) == 4
+
+
 @pytest.mark.django_db
 def test_get_filter_started_at(alert_group_internal_api_setup, make_user_auth_headers):
     user, token, _ = alert_group_internal_api_setup

engine/apps/api/views/alert_group.py

@@ -324,7 +324,7 @@ def get_serializer_class(self):
     def get_queryset(self, ignore_filtering_by_available_teams=False):
         # no select_related or prefetch_related is used at this point, it will be done on paginate_queryset.
 
-        alert_receive_channels_qs = AlertReceiveChannel.objects.filter(
+        alert_receive_channels_qs = AlertReceiveChannel.objects_with_deleted.filter(
             organization_id=self.request.auth.organization.id
         )
         if not ignore_filtering_by_available_teams: