isNodeUnreachable returned true for any non-gRPC-status error, so a reachable
pre-upgrade server's missing full_teardown_done ack (a plain error) was
classified unreachable and the unreported pair was silently skipped. Classify
only a real codes.Unavailable as unreachable, and wrap the missing ack in a
sentinel the sweep treats as fatal regardless. A genuinely down node still
surfaces as Unavailable from the RPC and stays best-effort.