Cart SDK Zod schemas drop fields from account checkouts and add-to-cart responses

[field123]

@epcc-sdk/sdks-cart-checkout-order/zod provides schemas for validating API payloads. Two of them silently remove data:

• zCheckoutApiBody.parse(body) on an account checkout removes data.account and data.contact.
• zCartsResponse.parse(response), used for the response to adding items to a cart, removes most item fields.

Expected: parsing a valid payload keeps all of its fields.

Cause:

• In the checkout schema, every field of the customer-checkout shape is optional, so an account checkout matches that shape first and its extra fields are stripped.
• In the cart schema, response items are described with the request item shapes.

The TypeScript types are correct; only the schemas are affected.