Quotes and creates a job in a single call, for partners that always commit to
whatever quote they receive.

The body is a POST /jobs body without quote_id or undeliverable_action —
a quote is created internally and consumed immediately, and undeliverable
items are always disposed. The response is either the quote rejection that a
POST /quotes call would have returned, or the created job.

Supply an Idempotency-Key header to make retries safe. A repeat of the same
key replays the original response — including a rejection — rather than
returning a conflict, so a partner wanting a genuine retry after a transient
rejection such as couriers_busy must send a new key. Reusing a key with a
different body returns 422 idempotency_key_reused.