Special logic for x-ms-mutability and possibly other cases doesn't work when a sibling of $ref

[mikekistler]

In developing the fix for Azure/azure-sdk-tools#6840, we discovered that the special logic in lib/swaggerValidator/ajvSchemaValidator.ts for x-ms-mutability doesn't work when that property is a sibling of "$ref".

https://github.com/Azure/oav/blob/develop/lib/swaggerValidator/ajvSchemaValidator.ts#L288

  // If a response has x-ms-mutability property and its missing the read we can skip this error
  if (
    cxt.isResponse &&
    ((keyword === "required" &&
      (parentSchema.properties?.[(params as any).missingProperty]?.[xmsMutability]?.indexOf(
        "read"
      ) === -1 ||
        // required check is ignored when x-ms-secret is true
        (parentSchema.properties?.[(params as any).missingProperty] as any)?.[xmsSecret] ===
          true)) ||
      (keyword === "type" && data === null && parentSchema[xmsMutability]?.indexOf("read") === -1))
  ) {
    return true;
  }

Autorest allows some properties to be siblings of $ref, and specifically any "x-" properties.

https://github.com/Azure/autorest/blob/main/docs/openapi/howto/%24ref-siblings.md