Snapshot review_agent._session_messages before teardown so close() can
clean per-session state without dropping the user-visible
self-improvement summary. Adds two regressions:
- bg-review summarizer receives captured review-agent tool messages
after review_agent.close() runs
- context-compressor protected-head handoff rehydration populates
_previous_summary and keeps the old handoff out of newly summarized
turns
Salvaged from PR #26039 onto current main after agent/background_review.py
extraction. Original commit 63eaf6055; bg-review test updated to patch
the module-level summarize_background_review_actions in
agent.background_review instead of the now-forwarder
AIAgent._summarize_background_review_actions.
