Muted alert started to send notifications to Slack

### Environment

SaaS (https://sentry.io/)

### Steps to Reproduce

1. Create an Issue Alert that sends Slack notifications on every new issue
2. Mute the alert
3. Wait a month (I hate how non-reproducible it is, I'm so sorry, but this is what happens to us)

Note: These issues are not new at all. The Sentry project has been creating and updating issues constantly all this time- it's just now that the alert started to trigger, and send notifications to Slack

### Expected Result

The alert doesn't send any notifications for as long as it stays muted

### Actual Result

We started to get Slack notifications out of the blue. Luckily it's an internal testing channel and it's not bothering the developers yet :)

Unmuting it and muting it again seems to have stopped them from coming, but I thought it's worth to report what happened either way.

### Product Area

Alerts

### Link

_No response_

### DSN

_No response_

### Version

_No response_

Start with the Issue Alert mute/unmute flow and the Slack notification delivery path described in the reproduction steps. Try to reproduce the delayed notification behavior, then verify that a muted alert suppresses Slack notifications while it remains muted and that the existing regression behavior is covered by a test.

Written by the indexing model from the issue text.