Cart metafields are currently unavailable in Shopify Function APIs for cart-checkout-validation extensions. A developer needs to access cart metafields within their extension but discovered no input option exists in the GraphQL schema. As a temporary workaround, cart attributes can be used instead of metafields until native support is added.
There is no input mention to get metafields stored on a cart
https://shopify.dev/docs/api/functions/reference/cart-checkout-validation/graphql/input
I need to access those metafields in this extension
https://shopify.dev/docs/apps/checkout/validation/server-side
Cart metafields are not (yet) available in Function APIs. You might try cart attributes as a workaround in the meantime.