Failed

the server's own error text, when the transport carried one (e.g. an HTTP error body's message) — null for a bucket with no such text (a timeout, a local cancellation, a bare non-2xx with no parseable body). Never a substitute for reason: a UI branches on reason, and may additionally show detail as a footnote.

the server-advised wait, from a Retry-After header, when present. Only ever set alongside reason == AiFailure.RateLimited in practice, but not constrained to that — a caller should treat null as "no guidance", not "wait forever".