Alerting: Deleting a Contact Point used in a Notification Policy leaves the Alertmanager in an inconsistent state

### What happened? When attempting to delete a Contact Point that is currently being used on a Notification Policy, the Grafana UI won't allow it. <img width="486" height="227" alt="Image" src="https://github.com/user-attachments/assets/4d854d45-1fa8-498d-ae6d-10266397cb75" /> However when using the API (provisioning) this is possible, and leaves the Alertmanager in a inconsistent state. Tested in: Grafana: Grafana v12.4.x (upgraded from previous version v11.6.9) (On-prem no Cloud) With existing default contact point: `grafana-default-email` / default NP <img width="1076" height="567" alt="Image" src="https://github.com/user-attachments/assets/74765c60-a977-4819-94d5-8cd8475ef4a2" /> <img width="634" height="165" alt="Image" src="https://github.com/user-attachments/assets/86bb7ce8-9de9-4516-8511-542cf7dd17d6" /> <img width="1261" height="292" alt="Image" src="https://github.com/user-attachments/assets/6f869f2c-f757-4224-a33e-d51433ef4064" /> Note: it does not happen on freshly new installs but those that were upgraded from older Grafana versions. Response from the .har when loading Contact Points page: `http://localhost:3000/apis/notifications.alerting.grafana.app/v0alpha1/namespaces/default/receivers` ``` { "kind": "Status", "apiVersion": "v1", "metadata": {}, "status": "Failure", "message": "Current Alertmanager configuration in the storage is corrupted. Reset the configuration or rollback to a recent valid one.", "reason": "InternalError", "details": { "uid": "alerting.notification.configCorrupted", "causes": [ { "message": "\"unexpected receiver (grafana-default-email) is undefined\"", "field": "Error" } ] }, "code": 500 } ``` Har: [alerting_cp_api_bug.zip](https://github.com/user-attachments/files/25101979/alerting_cp_api_bug.zip) ### What did you expect to happen? The API does not allow you to delete the Contact point and return 409 code ### Did this work before? Not sure about this point.…