A failed flush at on_session_switch() restored the pending buffer and
then cleared it on the next line, so an unavailable service at the switch
boundary still lost every pending turn. Pending turns now carry their own
session_id; _write_turns() batches per session, so a later retry (next
turn, session end, shutdown) writes old-session turns under the old
session's custom_id even after the switch.
Addresses review on #109359.
1b76cfff83