fix(compression): merge the todo snapshot into the trailing user turn

[teknium1]

Summary

After context compression, the preserved todo snapshot is no longer appended as a standalone synthetic {role:"user"} message that can read as a fresh user instruction — it is folded into the trailing real user turn instead (mirroring the summary merged-into-tail precedent), preserving role alternation and injection authority. Root cause: compress_context unconditionally appended the todo snapshot as a new user-role row after the compressed transcript, so the todo text became the tail user turn (#26979) and — when the tail was already a user message — created consecutive user/user turns some providers reject.

Changes

• agent/conversation_compression.py: fold the todo snapshot into a trailing user message when one exists (blank-line separated for strings, appended text block for multimodal lists via _append_text_to_content); fall back to the flagged standalone append otherwise (salvaged from fix(compression): merge todo snapshot into trailing user msg to avoid consecutive user/user turns #53890).
• agent/conversation_compression.py (follow-up hardening): merge only into REAL user tails — a _is_real_user_message probe keeps scaffolding tails (continuation marker, summary-as-user handoff, bare stale snapshot rows) on the flagged standalone path, because merging would bury the TODO_INJECTION_HEADER / summary-prefix content markers mid-content and upgrade those rows to "real user" evidence after SessionDB projection strips the underscore flags, breaking fix(compression): preserve zero-user provenance in compaction summaries #69292's zero-user provenance (_is_synthetic_compression_user_turn keys on those start-of-content markers).
• agent/conversation_compression.py: new _strip_stale_todo_snapshot — a snapshot merged at an earlier boundary is stripped before re-injection so repeated compactions refresh rather than accumulate todo state, and a bare stale snapshot row is refreshed in place instead of stacking a duplicate (empty/stale-skip semantics from Prevent todo snapshot injection after compression #26981; the empty-store skip itself lives in TodoStore.format_for_injection, which returns None for empty or all-completed lists — now pinned by test).
• tests/agent/test_compression_rotation_state.py: contributor tests (merge into trailing user, multimodal tails, in-place persistence, no user/user adjacency) plus follow-up tests (scaffolding tails stay standalone with provenance verified via _transcript_has_real_user_turn, stale-row refresh, strip-before-reinject, empty-store no-op).

Validation

	Before	After
Compressed transcript tail with active todos	Standalone synthetic user turn carrying todo text (reads as a fresh instruction; user/user adjacency when tail is user)	Snapshot folded into the trailing real user turn; alternation preserved
Tail is scaffolding (continuation marker / summary-as-user)	Merge would erase synthetic classification after DB projection	Flagged standalone append retained; _is_synthetic_compression_user_turn classifies both shapes; zero-user provenance (#69292) intact
Repeated compaction boundaries	Snapshots accumulate	Previous snapshot stripped/refreshed; exactly one snapshot block

Targeted tests: tests/agent/ -k 'todo and (snapshot or inject)' → 11 passed; tests/agent/test_context_compressor_zero_user_provenance.py → 10 passed; tests/agent/test_compression_rotation_state.py → 25 passed; tests/agent/test_context_compressor.py + tests/agent/test_compression_concurrent_fork.py + tests/run_agent/test_run_agent.py → 674 passed. 0 failed.

Note on the longer-term direction: #58162 by @0xLeathery (draft) proposes preserving todos as tool state (tool-call/tool-result pair) rather than any user-role row — the architecturally cleaner endgame. This PR keeps the user-visible fix shippable now without blocking that direction.

Credit

Salvaged from #53890 by @yingliang-zhang; empty-snapshot handling from #26981 by @YLChen-007; #58162 by @0xLeathery notes the longer-term tool-state direction. Fixes #26979.

Infographic

[github-actions]

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_{ran on 7eb7d0a}

all good!