curl "https://api.launchmystore.io/metafield-definitions/<id>" \
  -H "Authorization: Bearer <merchant-jwt>"

{
  "status": 200,
  "state": "success",
  "data": {
    "id": "f7e3a920-...",
    "namespace": "custom",
    "key": "warranty_years",
    "name": "Warranty (years)",
    "description": "How many years of warranty are included",
    "type": "number_integer",
    "owner_type": "product",
    "validations": { "min": 0, "max": 25 },
    "pinned": true,
    "app_id": null,
    "created_at": "2026-05-09T13:30:00.000Z",
    "updated_at": "2026-05-09T13:30:00.000Z"
  }
}