[BUG] Machine Learning Batch Endpoint API Spec documents unsupported identity types

### API Spec link

https://github.com/Azure/azure-rest-api-specs/blob/main/specification/machinelearningservices/resource-manager/Microsoft.MachineLearningServices/stable/2025-09-01/mfe.json#L13050-L13052

### API Spec version

2025-06-01, 2025-09-01

### Describe the bug

According to the API spec the API supports system assigned and/or user assigned identities. But passing a user assigned identity results in the following error:

Error when passing "SystemAssigned,UserAssigned":

```json
{
"error": {
"code": "CannotSetResourceIdentity",
"message": "Resource type 'Microsoft.MachineLearningServices/workspaces/batchEndpoints' does not support creation of 'SystemAssigned, UserAssigned' resource identity. The supported types are 'SystemAssigned'."
}
}
```

Error when passing "UserAssigned":

```json
{
"error": {
"code":" CannotSetResourceIdentity",
"message": "Resource type 'Microsoft.MachineLearningServices/workspaces/batchEndpoints' does not support creation of 'UserAssigned' resource identity. The supported types are 'SystemAssigned'."
}
}
```

This is causing issues downstream in an SDK generated from this API spec.

### Expected behavior

As batch endpoint only support `SystemAssigned` identity, the `identity` block should reference the right identity definition. Currently, it refers to:

```
"properties": {
"identity": {
"description": "Managed service identity (system assigned and/or user assigned identities)",
"$ref": "../../../../../common-types/resource-management/v3/managedidentity.json#/definitions/ManagedServiceIdentity"
}
```
Instead, it should be referring it.

```
"$ref": "../../../../../common-types/resource-management/v3/types.json#/definitions/Identity"
```

And the `description` should be updated accordingly.

### Actual behavior

Error as described above

### Reproduction Steps

```json
PUT https://management.azure.com/subscriptions/{subscriptionID}/resourceGroups/{resourceGroupName}/providers/Microsoft.MachineLearningServices/workspaces/{machineLearningWorkspaceName}/batchEndpoints/{batchEndpointName}?api-version=2025-09-01

{
"identity": {
"type": "SystemAssigned,UserAssigned",
"userAssignedIdentities": {
"identityId": {}
}
},
"location": "australiaeast",
"properties": {
"authMode": "AADToken",
"description": "Test batch endpoint"
},
"tags": {
"Environment": "Test"
}
}
```

### Environment

_No response_

Start with specification/machinelearningservices/resource-manager/Microsoft.MachineLearningServices/stable/2025-09-01/mfe.json around lines 13050-13052 and compare the identity reference with common-types/resource-management/v3/types.json. Check the corresponding 2025-06-01 definition as well; done means both batch endpoint API specs describe only system-assigned identity support and no longer advertise user-assigned identities.

Written by the indexing model from the issue text.