[AvoidAnonymousTypes] False positive on ManagedServiceIdentity generated from TypeSpec

## Problem
Rule `AvoidAnonymousTypes` raises a false-positive error on `UserAssignedIdentity` generated from TypeSpec.

### TypeSpec
```
...ManagedServiceIdentityProperty;
```

### Swagger
```
"Azure.ResourceManager.CommonTypes.ManagedServiceIdentityUpdate": {
"type": "object",
"description": "Managed service identity (system assigned and/or user assigned identities)",
"properties": {
"type": {
"$ref": "../../../../../common-types/resource-management/v5/managedidentity.json#/definitions/ManagedServiceIdentityType",
"description": "The type of managed identity assigned to this resource."
},
"userAssignedIdentities": {
"type": "object",
"description": "The identities assigned to this resource by the user.",
"additionalProperties": {
"allOf": [
{
"$ref": "../../../../../common-types/resource-management/v5/managedidentity.json#/definitions/UserAssignedIdentity"
}
],
"type": "object",
"x-nullable": true
}
}
}
},
```

## Workaround
Suppress in `readme.md` until root cause is addressed

## Example
https://github.com/Azure/azure-rest-api-specs-pr/pull/19710/checks?check_run_id=30708435029

## Related
https://github.com/Azure/typespec-azure/issues/1058.