Mint {upload_id, storage_path, put_url, token_expires_at, max_bytes} for one source.

Costs nothing quota-wise. Call before POST /v1/runs when a local file must be uploaded, then execute the signed PUT directly and probe readiness. Recover from unsupported_media_type by re-encoding to an allowed extension, upload_too_large by using a supported source_url, and validation_error by fixing the named field. Retry service_unavailable after retry_after_sec.