[Breaking Change] Rule "AddedXmsEnum" treats "string" and "open enum" as incompatible

Given the following types:

```
// string
"type": "string"

// closed enum
"type": "string",
"x-ms-enum": { "modelAsString": false },
"enum": [ "westus", "westeurope"]

// open enum
"type": "string",
"x-ms-enum": { "modelAsString": true },
"enum": [ "westus", "westeurope"]
```

Types "string" and "open enum" should be considered fully compatible, since "open enum" is just documenting the valid values and saying that the set is open. "closed enum" should be considered incompatible with both "string" and "open enum", since the valid values are closed.

In https://github.com/Azure/azure-rest-api-specs/pull/26658, check "Swagger BreakingChange" seems to get this wrong in rule and `1048 - AddedXmsEnum`.


```
// from
"type": "string",
"description": "The type discriminator describing a sub-type of QueueSelectorAttachment"

// to
"type": "string",
"description": "Supported queue selector attachment types",
"enum": [ "conditional", ... ],
"x-ms-enum": { "modelAsString": true ... }
```