[BUG] Metric Alerts does not return properties.autoMitigate when using default

### API Spec link

https://github.com/Azure/azure-rest-api-specs/blob/main/specification/monitor/resource-manager/Microsoft.Insights/stable/2018-03-01/metricAlert_API.json

### API Spec version

2018-03-01

### Describe the bug

Hi,
it seems then properties.autoMitigate is only returned by get/create or update when explicitly set to either true or false. It defaults to true server side when unset in a create/update so I'd say you should return it with its value, even if not explicitly set by create / update.

### Expected behavior

properties.autoMitigate returned always

### Actual behavior

properties.autoMitigate returned only when explicitly set to true/false by user

### Reproduction Steps

Create a metrics alert without properties.autoMitigate and then look at the returned object or do a seperate get on the newly created object.

### Environment

_No response_

Start with the linked specification/monitor/resource-manager/Microsoft.Insights/stable/2018-03-01/metricAlert_API.json definition and inspect how properties.autoMitigate is represented. Confirm that the API specification captures the expected always-returned behavior when the server-side default is used, and verify the definition against the reported create, update, and get scenarios.

Written by the indexing model from the issue text.