Fix: Handle $ref as JSON object in OpenAPI specifications

Problem

The ObjectPath.FromObject() method assumes that $ref fields in OpenAPI specifications are always strings (references like "#/definitions/FieldType"). However, in some OpenAPI specs, $ref can be a JSON object that defines a schema, causing an InvalidCastException when the code attempts to cast a JObject to a string.

This showed up in the CI run for the following azure-rest-api-specs error below:
https://github.com/Azure/azure-rest-api-specs/actions/runs/18208997652

Reproduction Case

When processing OpenAPI specifications containing schema definitions like:

"FieldDefinition": {
  "type": "object",
  "properties": {
    "$ref": {
      "type": "string",
      "description": "Reference to another field definition."
    }
  }
}

The following error occurs:

System.InvalidCastException: Cannot cast Newtonsoft.Json.Linq.JObject to Newtonsoft.Json.Linq.JToken.
at Newtonsoft.Json.Linq.Extensions.ConvertT, U
at Newtonsoft.Json.Linq.Extensions.ValueT, U
at OpenApiDiff.Core.Logging.ObjectPath.FromObject(JObject o, String name)

Root Cause

The original code in FromObject() method attempted to call @ref.Value<string>() without checking if @ref is actually a string. When $ref is a JSON object (schema definition), this causes a cast exception.

Solution

• Added type checking to verify @ref.Type == JTokenType.String before attempting string conversion

Testing

• ✅ Run npm run dn.test and verified its pass
• ✅ Confirmed no regression in existing functionality for string-based $ref values
• ✅ Tested with the original repro case OpenAPI specifications containing schema definitions in $ref fields