a.Child keyword-only: z
a.Child {"ref": {"back": {"ref": {}}}, "z": 1}: schema accepts, model accepts
a.Child {"ref": {}}: schema rejects, model rejects (missing)
a.Child {"z": 1}: schema rejects, model rejects (missing)
a.Child {"ref": {}, "z": "one"}: schema rejects, model rejects (int_parsing)
