When a limit is reached the API answers
429 RATE_LIMITED with Retry-After. Organizations have no limit of their own
on runs in flight or waiting. If the whole service has too many runs waiting, new runs get 503 SERVICE_UNAVAILABLE with
Retry-After, before anything is held; retry with the same Idempotency-Key. See Errors.
Spend is limited separately: give each agent its own key with a spend cap (Pricing and billing).