Pre-submit validation for a Basket

Runs the same validation suite that POST /shop/basket/{basketId}/submit runs — checking availability, customer details, payment requirements, and integration pre-conditions — without committing the basket. The response is the latest basket payload, with any validation failures surfaced via the submit_errors collection.

Call this immediately before the customer enters payment details so you can fail fast and avoid taking money for a basket that's going to be rejected on submit.

Returns 400 Bad Request if the basket can't even reach the payment step (e.g. an item is unavailable or a payment processor error occurred during pre-flight).