Re-queues failed items of a sample generation so they run again.

When no item ids are supplied, every failed item of the generation qualifies. Item ids that do not belong to the generation or are not in a failed state are ignored. Returns the ids that were re-queued; a generation with no failed items yields an empty result. When no failure categories are supplied, the retry covers only the items whose failure a re-run can plausibly fix, so items rejected for the request itself — an unknown model, refused credentials, a content-policy or configuration rejection — are left alone rather than re-run to collect the same rejection. Naming those categories explicitly retries them anyway; combining categories with item ids intersects the two.