[DSIP-81][Alert] Add Integration test for alert server.

### Search before asking

- [X] I had searched in the [DSIP](https://github.com/apache/dolphinscheduler/issues/14102) and found no similar DSIP.

### Motivation

todo:

### Design Detail

todo:

### Compatibility, Deprecation, and Migration Plan

todo:

### Test Plan

todo:

### Code of Conduct

- [X] I agree to follow this project's [Code of Conduct](https://www.apache.org/foundation/policies/conduct)

The issue identifies the alert server as the subject and requests an integration test, but provides no files, test entry points, design, or test plan. Start by locating the alert server's existing integration-test setup; done means the requested integration test is implemented and passes.

Written by the indexing model from the issue text.