Review feedback: the previous test called _mark_session_committed
directly, so it verified the guard's behavior but not the wiring that
sets it — a future break in the commit_memory_session -> same-id
compression-boundary path would not be caught.
Add a lifecycle regression that drives the real sequence: on_session_end
commits through the actual path, on_session_switch(same id,
reason="compression") crosses the boundary, sync_turn records a genuinely
new turn, and a second on_session_end must produce a second commit POST.
Without the fix it fails showing exactly one commit call, which is the
reported data loss: every turn after the first compression is dropped.
The rotation and /undo tests stay as scope guards.
(cherry picked from commit 0ca5a330630a30b105cbbc32e8a23f2c5ffe0eab)
Jeff Mettel
f94914f773