A response arriving during a hold was sent before it began, so a late
success no longer releases the host; only one after the hold has ended
does.
Artifact fetches check RateLimitGate.Held before calling the fetcher, so
held cache misses fail with the HeldError without counting as failures
in the host's circuit breaker.
Every handler that answers an upstream fetch error with 502 now answers
a held request with 503 and Retry-After instead.
OnHold and RecordUpstreamRateLimitHold drop the unused duration.
A 429, or a 503 with Retry-After, from an upstream host now stops the
proxy sending that host anything for as long as it asked (capped at an
hour), or for a minute doubling up to fifteen when no Retry-After came.
Held requests fail locally: cached metadata is served stale as before,
and anything uncached answers 503 with the remaining Retry-After so the
client paces itself too.
The gate is a RoundTripper under the auth transport, so metadata,
artifact and passthrough fetches all obey it. Metadata fetches had no
retry or circuit breaker, so a rate-limited upstream kept receiving one
request per cache miss for the whole block. /health lists held hosts
under upstream_holds and proxy_upstream_rate_limit_holds_total counts
holds.
