✅(backend) fix auth unit test with django-lasuite 0.1.16 update

django-lasuite 0.1.16 changed the user update mechanism from .update()
to .save(), which triggers Django's constraint validation. This causes
an additional SELECT query to verify 'sub' field uniqueness on every
user update, despite 'sub' being immutable in our auth flow.

This commit update the test to make them pass again.

src/backend/core/tests/authentication/test_backends.py

@@ -345,7 +345,9 @@ def test_authentication_getter_existing_user_change_fields(
     monkeypatch.setattr(OIDCAuthenticationBackend, "get_userinfo", get_userinfo_mocked)
 
     # One and only one additional update query when a field has changed
-    with django_assert_num_queries(2):
+    # Note: .save() triggers uniqueness validation queries for unique fields,
+    # adding extra SELECT queries before the UPDATE (e.g., checking unique=True on 'sub')
+    with django_assert_num_queries(3):
         authenticated_user = klass.get_or_create_user(
             access_token="test-token", id_token=None, payload=None
         )