fix(compression): persist rotated session snapshot immediately

[LeonSGP43]

Summary

• persist the compressed transcript immediately after a compression-driven session split
• advance the new session flush cursor so later end-of-turn persistence does not duplicate writes
• add a regression test that reads the rotated session immediately and verifies the latest assistant turn is already present

Testing

• uv run --frozen pytest -q -o addopts= tests/run_agent/test_compression_boundary_hook.py tests/run_agent/test_compression_persistence.py
• uv run --frozen ruff check agent/conversation_compression.py tests/run_agent/test_compression_boundary_hook.py
• git diff --check

Fixes #29522

[teknium1]

Thanks for this, and for the clean regression test — the bug you targeted was real when you filed it.

Closing because the premise has since been superseded on main, and the linked issue (#29522) is already resolved a different way:

• compression.in_place is now the default (True) — PR feat(compression): in-place compaction option, single session id (#38763) [1/2] #49739 / feat(compression): flip in_place default to True + guard fix (#38763) [salvage #51959] #52658 (fix(compression): eliminate session duplication -- adopt in-place compaction like Claude Code and Codex #38763). Compaction rewrites the transcript and rebuilds the system prompt while keeping the same session id — no fork to a child session. That removes the rotated-but-empty-child window your patch closes; there's no new session_id for Workspace to refetch before the next flush. Your fix lands on the legacy else (rotation) branch that most users no longer hit.
• The agent-side half of Automatic context compaction can hide or drop just-completed assistant response #29522 is fixed by _ensure_last_assistant_message_in_tail (agent/context_compressor.py:2274, invoked on every compaction at :2479) — it pulls the cut point back so the most recent assistant reply always stays in the protected tail and is never rolled into the summary.
• Automatic context compaction can hide or drop just-completed assistant response #29522 was closed COMPLETED on this basis; the only remaining thread was the Workspace UI reading the active set correctly, tracked out-of-repo at Assistant response disappears after automatic context compaction outsourc-e/hermes-workspace#505.

So this isn't a quality issue — it's a correct fix for a code path the codebase deliberately moved past. Closing as superseded. Appreciate the contribution and the well-scoped test.