The marker-preservation test is subsumed by the _db_flush_collect
regression test and the updated exact-dict assertion in
test_cached_agent_history_guard.py; keep one invariant test. Also note
why _build_replay_entry carries the stamp: replay rewrites are view-only
and marker-only flushes must skip rows already persisted.
Co-authored-by: finn763 <165816600+finn763@users.noreply.github.com>
Co-authored-by: Gaurav Saxena <gauravsaxena.jaipur@gmail.com>