reject empty schedule_days with 422

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

tests/api/test_scheduler_endpoints.py

@@ -252,18 +252,17 @@ class TestUpdateSchedulerConfig:
         assert response.status_code == 422
 
     @pytest.mark.asyncio
-    async def test_empty_schedule_days_accepted(
+    async def test_empty_schedule_days_rejected(
         self, authenticated_client, mock_config_service, mock_scheduler_service
     ):
-        """Empty schedule_days list is valid (disables the cron job)."""
+        """Empty schedule_days list is invalid and returns 422."""
         payload = {"enabled": True, "schedule_days": []}
 
         with patch("src.server.api.scheduler.get_config_service", return_value=mock_config_service), \
              patch("src.server.api.scheduler.get_scheduler_service", return_value=mock_scheduler_service):
             response = await authenticated_client.post("/api/scheduler/config", json=payload)
 
-        assert response.status_code == 200
-        assert response.json()["config"]["schedule_days"] == []
+        assert response.status_code == 422
 
     @pytest.mark.asyncio
     async def test_update_enable_disable_toggle(