[BUG] MongoCluster API responds with HTTP 500 when changing from no identity to user assigned identity

### API Spec link

[azure-rest-api-specs/specification/mongocluster/resource-manager/Microsoft.DocumentDB/MongoCluster/stable/2025-09-01/mongoCluster.json](https://github.com/Azure/azure-rest-api-specs/blob/b7943b8282c0d50ecc6197c5582d2bc4797415dc/specification/mongocluster/resource-manager/Microsoft.DocumentDB/MongoCluster/stable/2025-09-01/mongoCluster.json#L2034-L2037)

### API Spec version

2025-09-01

### Describe the bug

Unable to add UAI to existing cluster

### Expected behavior

Able to add UAI to existing cluster, or API responds with an informative error message rather than `Internal server error`

### Actual behavior

API responds with 500 internal server error:

```
{
"error": {
"code": "internal_server_error",
"message": "Internal server error."
}
}
```

### Reproduction Steps

1. Create a cluster using the API without adding the `identity` object.
2. Update the cluster, adding the `identity` object

### Environment

_No response_

Start with the referenced specification/mongocluster/resource-manager/Microsoft.DocumentDB/MongoCluster/stable/2025-09-01/mongoCluster.json around lines 2034-2037, then reproduce the update by creating a cluster without identity and adding a user-assigned identity. Done means the update succeeds or returns an informative error instead of HTTP 500.

Written by the indexing model from the issue text.