[Breaking Change] Cross-Version Runtime Exception: Error: Error retrieving property

The Breaking Changes (Cross-Version) check is failing in [PR 24387](https://github.com/Azure/azure-rest-api-specs/pull/24387) with the error:

> "details":"Command failed: node "/mnt/vss/_work/_tasks/AzureApiValidation_5654d05d-82c1-48da-ad8f-161b817f6d41/0.0.54/common/temp/node_modules/.pnpm/@Azure+oad@0.10.4/node_modules/autorest/dist/app.js" --v2 --input-file=specification/cdn/resource-manager/Microsoft.Cdn/preview/2023-07-01-preview/afdx.json --output-artifact=swagger-document.json --output-artifact=swagger-document.map --output-file=new --output-folder=/tmp\nFATAL: swagger-document/emitter - FAILED\nFATAL: Error: Error retrieving '$.definitions.UrlSigningAction.type' (Error: Trying to retrieve 'type' from mapping that contains no such key)\n Error: Error retrieving '$.definitions.UrlSigningAction.type' (Error: Trying to retrieve 'type' from mapping that contains no such key)\n"

GH check:
- https://github.com/Azure/azure-rest-api-specs/pull/24387/checks?check_run_id=14698482189

The message part of this
> Error retrieving '$.definitions.UrlSigningAction.type'

suggests that this is a problem in the REST API definition -- that the UrlSigningAction schema is missing a `type` field. But that is not the case.

```
"UrlSigningAction": {
"description": "Defines the url signing action for the delivery rule.",
"type": "object",
"x-ms-discriminator-value": "UrlSigning",
"allOf": [
{
"$ref": "./cdn.json#/definitions/DeliveryRuleAction"
}
],
"required": [
"parameters"
],
"properties": {
"parameters": {
"description": "Defines the parameters for the action.",
"$ref": "#/definitions/UrlSigningActionParameters"
}
}
},
```