details.quota.metric in the structured error that rejects a request;
the run-rate and volume limits also read back from hiloop usage.
quota_exceeded, and details.quota.metric says
which one. A request over a rate limit uses rate_limited and includes Retry-After. Preserve the
same idempotency key and retry with bounded backoff.
Delete unneeded volume versions to release storage quota. Stopping a sandbox frees a running slot;
only deleting it frees a total slot.