Two hardening fixes prompted by review on #28296:
1. sync_turn() now snapshots the target session id before spawning the
worker. The previous code read self._session_id inside the worker, so
a worker delayed past on_session_switch's bounded join could read the
rotated-in NEW id and write the OLD turn's messages into the wrong
session.
2. on_session_end() resets _turn_count to 0 after a successful commit,
making the old-session commit path idempotent with the new switch
hook. /new and compression call commit_memory_session() (which fires
on_session_end) immediately before on_session_switch; without this,
the old session would be committed twice. On commit failure we leave
_turn_count > 0 so on_session_switch retries.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
(cherry picked from commit 2ea8d5c537bccad814c483d36ddd79904bf6b55c)
a30b40c73a