Security.AssessmentMetadata API always returns `Unknown` when `categories` isn't specified in the request body

After tested, seems Security.AssessmentMetadata API always returns `Unknown` when [`categories`](https://github.com/Azure/azure-rest-api-specs/blob/c059fb47f87a35ce5a90631883dfba8f01a41d8e/specification/security/resource-manager/Microsoft.Security/stable/2020-01-01/assessmentMetadata.json#L317) isn't specified in the request body. But “Unknown” isn't in the category list. So it doesn't make sense. After synced with service team, they confirmed will add `Unknown` as possible value to this property and it will be published as a new version of this API.

Http request body:
```
PUT https://management.azure.com/subscriptions/xx-xx-xx-xx/providers/Microsoft.Security/assessmentMetadata/xxx-xxx-xxx-xxx?api-version=2020-01-01

{
"properties": {
"displayName": "xxx-xxx-xxx-xxx",
"description": "Test description",
"severity": "Medium",
"assessmentType": "CustomerManaged",
}
}
```

Http response:
```
{
"id": "/subscriptions/xxx-xxx-xxx-xxx/providers/Microsoft.Security/assessmentMetadata/xxx-xxx-xxxx-xxx",
"name": "xxx-xxxx-xxxx-xxxx",
"type": "Microsoft.Security/assessmentMetadata",
"properties": {
"displayName": "xxx-xxxx-xxxx-xxxx",
"assessmentType": "CustomerManaged",
"description": "Test Description",
"categories": [
"Unknown"
],
"severity": "Medium"
}
}
```

Start with specification/security/resource-manager/Microsoft.Security/stable/2020-01-01/assessmentMetadata.json, especially the categories definition linked in the issue. Compare the declared category values with the documented response behavior and the service team's stated API-version update. Done means the specification accurately represents Unknown for responses when categories is omitted, with validation passing.

Written by the indexing model from the issue text.