Get the current state of a cart

Reads back a cart’s lines, totals and checkoutUrl by id. Use it to re-render the basket on load, or to restore a returning shopper’s basket from a saved cart id. It only reads the cart — it never adds, removes or re-prices items. A cart a member token has bound (metadata.member_key) is read only with that member’s token, and its checkoutUrl comes back without the ct ticket, which the hosted checkout refuses: send the member to the checkoutUrl from the latest member-token call on a binding route (create, PATCH /api/cart, /identity, /lines), or make one — e.g. PATCH /api/cart with the current locale — to get a fresh one.