Queue 1 to 100 validations as one batch

Accepts 1 to 100 jobs together, all or none: a job that would be refused refuses the batch, and the Problem's detail names it by position (Job 3: ...), as errors[].field does (jobs[3].options). The documents together are at most 25 MiB (26,214,400 bytes) and the envelope at most 36 MiB. PDFs are inspected on acceptance, within the 30-second hybrid deadline for the whole batch; a batch that cannot be is 504 validation_timeout and nothing is queued, so send fewer PDFs at once. Each job then runs, is charged and is retained on its own.

Idempotency. An Idempotency-Key names the whole batch: the same key and envelope return the original batch (200), a different envelope 409 idempotency_conflict. Keys are V2's own, apart from V1 batches and from synchronous validation.