New `owner_type` field for metafield declarations in Checkout and Customer Account UI extensions, required in 2027-01

Starting with API version 2026-10 , Checkout and Customer Account UI extensions can specify an owner_type for each metafield declaration in shopify.extension.toml . Starting with API version 2027-01 , owner_type is required for every metafield declaration, and deployments without it will be blocked. What changed Metafield declarations in [[extensions.metafields]] and [[extensions.targeting.metafields]] accept a new owner_type property.