fix(compressor): skip context-summary markers as last-user tail anchor

[teknium1]

Summary

Context compaction no longer mistakes its own handoff banner for the latest user turn. A [CONTEXT COMPACTION — REFERENCE ONLY] summary is inserted with role="user" when the protected head ends in an assistant/tool message; on a resumed or multi-compaction session, _find_last_user_message_idx returned that banner as the "latest user message", so _ensure_last_user_message_in_tail anchored the tail to the summary and rolled the genuine last user turn into the next compaction — the exact active-task loss the anchor exists to prevent (#10896 / #22523).

Root cause: _find_last_user_message_idx returned any role="user" message. The fix reuses the existing _is_context_summary_content helper (already used by _find_latest_context_summary and the summarizer serialization) to skip summary banners.

Changes

• agent/context_compressor.py: _find_last_user_message_idx skips context-summary markers when locating the last real user turn.
• tests/agent/test_compressor_assistant_tail_anchor.py: adds TestFindLastUserMessageIdxSkipsSummaryMarker (skips user-role summary; returns real user when no summary; -1 when all user messages are summaries).

Validation

	Before	After
last-user anchor with a user-role summary banner present	anchors to the summary → real user rolled into compaction (task loss)	anchors to the real user turn → protected in tail
targeted tests	—	24/24 test_compressor_assistant_tail_anchor.py green
compression suites	—	139/139 (test_context_compressor.py + test_compression_boundary.py) green
E2E	—	fixed path anchors real user (idx 2); simulated old path anchors summary (idx 4)

Salvage note

Salvaged from #36626 by Frank Song (issue #36624), authorship preserved. The PR's other two proposed changes are superseded on current main and were intentionally dropped:

• Demoting completed tool results inside the protected tail → covered by the min_tail floor _MAX_TAIL_MESSAGE_FLOOR=8 (fix(agent): preserve recent turns during compression #39170) and no-op compression counting at compress_start >= compress_end ([Bug]: Infinite Context Compaction Loop (messages=N->N) on low context_length / limit configurations #40803); demoting the tail the model is about to read is unnecessary and risky.
• Preflight compression_exhausted terminal result → main already returns compression_exhausted on the 413 path and an explicit "auto-compaction disabled" terminal error.

Infographic

— Nous Research

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This was generated by AI during triage.

Salvage of #36626 (issue #36624) by Frank Song — related, not a duplicate. Related to the compaction tail-anchor family: merged #45249 and closed #29862.