Bug Fix: Nested Chords Fail When Using django-celery-results with a Redis Backend (celery#9950)

* ~pass in app when calling GroupResult.restore in RedisBackend > on_chord_part_return

* ~updated unit test to align with bug fix to RedisBackend

Author: kumuthu53

celery/backends/redis.py

@@ -540,7 +540,7 @@ def on_chord_part_return(self, request, state, result,
                 callback = maybe_signature(request.chord, app=app)
                 total = int(chord_size_bytes) + totaldiff
                 if readycount == total:
-                    header_result = GroupResult.restore(gid)
+                    header_result = GroupResult.restore(gid, app=app)
                     if header_result is not None:
                         # If we manage to restore a `GroupResult`, then it must
                         # have been complex and saved by `apply_chord()` earlier.

t/unit/backends/test_redis.py

@@ -1285,7 +1285,7 @@ def test_on_chord_part_return(
         self.b.client.zrange.assert_not_called()
         self.b.client.lrange.assert_not_called()
         # Confirm that the `GroupResult.restore` mock was called
-        complex_header_result.assert_called_once_with(request.group)
+        complex_header_result.assert_called_once_with(request.group, app=self.b.app)
         # Confirm that the callback was called with the `join()`ed group result
         if supports_native_join:
             expected_join = mock_result_obj.join_native