The exec stream can truncate after busybox's 4294967 ms RTT timer-wrap
artifact, dropping the summary line — docker_ping then reported a
phantom 100 % loss and made the corrupt-filter scenario fail spuriously.
Factor the parsing into _busybox_ping_parse: the summary stays
authoritative when present; otherwise the loss is derived from the
seq= replies actually seen (no replies at all still reports 100 %).