Create Product Variant

curl -X POST 'http://localhost:9000/admin/products/prod_01HXYZABCDEF/variants' \
  -H 'Authorization: Bearer <token>' \
  -H 'Content-Type: application/json' \
  -d '{"title": "L", "sku": "LS-L", "options": {"Size": "L"}}'

const { product } = await client.admin.products.$id.variants.mutate({
  $id: "prod_01HXYZABCDEF",
  title: "L",
  sku: "LS-L",
  options: { Size: "L" },
})

{
  "product": {
    "id": "prod_01HXYZABCDEF",
    "title": "Linen Shirt",
    "variants": [
      { "id": "variant_01HXYZABCDEF", "title": "M", "sku": "LS-M" },
      { "id": "variant_01HXYZABCDEG", "title": "L", "sku": "LS-L" }
    ]
  }
}