When the per-session pending transcript queue hits _MAX_PENDING_PER_SESSION
(200) while the session DB is broken, the gateway previously popped the
oldest message and discarded it permanently — silent user data loss during
live operation (#78182). The on-disk pending spool only ran at shutdown via
flush_pending_to_file.
Extend that existing spool machinery for runtime drops:
- gateway/shutdown_flush.py: add spool_dropped_transcript_message() and
drain_transcript_spool(), reusing _get_flush_dir/_write_payload (same
atomic-JSON pending_messages/ spool format). recover_pending_to_db()
now also replays transcript_cap_drop payloads left over across restarts.
- gateway/session.py: on cap eviction, spool the dropped message and log a
WARNING that includes the spool path; if spooling fails, degrade to the
previous drop-and-warn behavior. On the next fully successful transcript
flush for that session, drain and replay spooled messages in drop order;
replay failures keep the spool files for the next attempt.
- tests/gateway/test_pending_queue_spool.py: drop→spool→drain roundtrip,
per-session drain isolation, spool-failure degradation, replay-failure
retention, and spool primitive ordering/reason filtering.
No new config; extends existing flush_pending_to_file infrastructure per
AGENTS.md guidance.
Refs #82616, #78182
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