`TestHandleListSessionsAddsSiblingTransfersWithoutChangingSessionShape`
expected `last_served_at` to equal `Session.LastActivityAt`, encoding the
client-progress fallback that fc65dcaf removed. The admin session list is
built from the same `streammonitor.LiveLocalSessions` projection as the
enforcer, so a never-served session now omits the field instead of reporting
the client's last progress report as a serve time.
The field is omitted for want of a server-observed serve, not removed: the
test now also asserts it reappears once a transport actually serves.
Follow-up to fc65dcaf, which I verified without running ./internal/api/...
Part of #305