The legacy completions API: each prompt runs the chat path as one user message (_serve), with the same checks,
task resolution, cascade, quota and storage as /v1/chat/completions; n, stream, stop, max_tokens and the rest
are the chat fields of the same name. A batch of prompts needs that many requests of the daily quota, checked before
the first runs; a batch that stops after its first prompt is an error that names the prompts already answered and
carries X-Should-Retry: false (_batch_stopped). A suffix, echo or best_of that would change the answer,
and token-array prompts, are a 400.