Artifact publishing recognizes contract-rollout and generic rate-limit errors

What

Artifact publishing error handling gained two refinements:

• Generic 429 ("too many requests") responses, beyond the existing capacity-cap case, are now classified as rate_limited and shown with a retry-after message
• A new contract_rollout error code is added for 422 responses whose message matches a "is newer than the preferred" pattern, but only when the request declared contract:'latest' (the latestContract flag) and there's no already-known contract pin. This is treated as distinct from the existing contract_echo_rejected case

Why

These give users and logs a more accurate reason when a publish fails: a genuine rate limit versus a contract version that's being rolled out ahead of what the client expected, rather than lumping both into generic or misleading error codes.