[BUG] Missing channels Property in Activity Logs REST API Specification

### API Spec link

https://github.com/Azure/azure-rest-api-specs/blob/05cfd9ee0ccb1825b042a0da182944d9630cfc55/specification/monitor/resource-manager/Microsoft.Insights/stable/2015-04-01/activityLogs_API.json#L148

### API Spec version

2015-04-01

### Describe the bug

The channels property is returned in the response when querying Azure Monitor Activity Logs via the REST API, but it is missing from the official api specification used to generate SDKs (e.g., Azure.ResourceManager.Monitor version 1.3.1). As a result, the SDK does not expose this property.

Reference to the spec:
[activityLogs_API.json#L148](https://github.com/Azure/azure-rest-api-specs/blob/05cfd9ee0ccb1825b042a0da182944d9630cfc55/specification/monitor/resource-manager/Microsoft.Insights/stable/2015-04-01/activityLogs_API.json#L148)

This issue was also raised in the Azure SDK for .NET repository:
https://github.com/azure/azure-sdk-for-net/issues/51348

The SDK team confirmed that the channels property is missing from the REST API specification and advised contacting the service team via this repository. Updating the spec would allow the SDK to surface this important property in future releases.

### Expected behavior

The channels property should be included in the API specification so that SDKs generated from it (like Azure.ResourceManager.Monitor) also expose this field. This property is important for filtering or analyzing logs by type (Admin, Operation, etc.).

### Actual behavior

The channels field is returned by the REST API response, but it is not documented in the API specification. Consequently, the SDK omits this field, limiting functionality for users relying solely on the SDK.

### Reproduction Steps

Use the REST API endpoint:
GET https://management.azure.com/subscriptions/{subscriptionId}/providers/microsoft.insights/eventtypes/management/values?api-version=2015-04-01

- Observe that the JSON response includes a channels property.
- Check the corresponding OpenAPI spec and note that channels is not defined.
- Attempt to access the channels property using Azure.ResourceManager.Monitor SDK — it is not available.

### Environment

_No response_

Start with specification/monitor/resource-manager/Microsoft.Insights/stable/2015-04-01/activityLogs_API.json at the referenced response schema and compare it with the documented REST response. Verify the GET activity-log endpoint response and the missing channels field. Done means the specification defines channels so generated SDKs can expose it.

Written by the indexing model from the issue text.