The public-API go-client (search/popular/etc.) had no mirror failover while
the agent control-plane client did — a primary-domain takedown broke public
calls. Inject a MirrorRoundTripper that reuses the SAME MirrorPool type +
IsTransient policy, rotating to cfg.Auth.Mirrors on a transient error/5xx.
WithRetry(0) hands failover ownership to the transport (no nested retry).
