Reports that the POS will not fulfil the order.

failure_code is the outcome the ordering channel is told, and is what the channel
maps onto its own reject vocabulary — always send the code that describes what
actually happened. info is the one line a channel shows a human.

A request that omits failure_code is still accepted, but only so that callers not
yet on this contract keep working: their reason and info are normalised onto a
canonical code on a best-effort basis. That tolerance is deprecated — send the code.

A channel fails an order over to another acceptance mechanism unless
store_explicitly_declined_order is true, so send that flag only for a fulfillable
order a person at the store deliberately turned down. Every automatic failure —
an offline POS, a timeout, a menu or payload problem — leaves it false.

Name the offending items in item_metadata whenever the code is about specific
items. The ids are resolved back to the ordering channel's own item ids, so a
channel can tell the customer which item was the problem.

Rejecting an order is idempotent, and cannot contradict an accepted one: a repeat
call refreshes the recorded detail, and an order already accepted or cancelled
answers 409.