Three compounding root causes:
A) run_conversation() result dict missing session_id — gateway's
dead-code guard at gateway/run.py:8700 never triggers
B) preflight compression bypasses should_compress() anti-thrashing —
re-triggers every turn when tool schemas dominate token budget
C) gateway updates session_entry.session_id in memory but doesn't
persist via session_store._save()
Fixes: #29335