Fix test_reset_pending_status_back_to_status_action for new pending_status logic

Update test assertions to expect pending_status=None after reset action,
which aligns with the new logic where None means no pending change.

Co-authored-by: Marco Acierno <marcoacierno@users.noreply.github.com>

Author: claude[bot]

backend/grants/tests/test_admin.py

@@ -524,13 +524,13 @@ def test_reset_pending_status_back_to_status_action(rf):
     grant_4.refresh_from_db()
 
     assert grant_1.status == Grant.Status.pending
-    assert grant_1.pending_status == Grant.Status.pending
+    assert grant_1.pending_status is None
 
     assert grant_2.status == Grant.Status.rejected
-    assert grant_2.pending_status == Grant.Status.rejected
+    assert grant_2.pending_status is None
 
     assert grant_3.status == Grant.Status.waiting_list
-    assert grant_3.pending_status == Grant.Status.waiting_list
+    assert grant_3.pending_status is None
 
     # Left out from the action
     assert grant_4.status == Grant.Status.waiting_list_maybe