The dispatcher's rehydrate callback reaches whichever backend answers the
API's public hostname. A backend with the runtime flag on that is not behind
that hostname, or the only one in the fleet with it on, could never get its
scripts deployed that way — the callback answered "disabled". The invoking
backend already knows the app and script, so on a dispatcher miss it deploys
the set itself and retries once, telling the dispatcher to skip its callback
and negative cache. The callback stays the path for evicted scripts.