Alerting: Notification policy timing options don't behave as expected

### What happened? The notification is sent out every 15 minutes ### What did you expect to happen? The notification to be sent out every 10 minutes. ### Did this work before? No idea, I hope so. ### How do we reproduce it? This is the notification policy timing options I have set up: <img width="720" height="437" alt="Image" src="https://github.com/user-attachments/assets/e27d6684-b732-4bec-924d-bcb24c6f3a4d" /> (I have **Group by** set to `Disable (...)`, so **Group wait** is set to 0s) When an an alert rule instance is firing, I would expect the notification to be repeated every 10 minutes (after the initial delay of 5 minutes), but it seems that **Group interval** is always added to **Repeat interval**. <img width="436" height="151" alt="Image" src="https://github.com/user-attachments/assets/a0fd9b66-c566-4a87-8ecd-564deb2aa54d" /> ### Is the bug inside a dashboard panel? _No response_ ### Environment (with versions)? Grafana: 12.2 OS: Browser: ### Grafana platform? Kubernetes ### Datasource(s)? _No response_