fix(anthropic): fold the per-call runtime addendum into the preceding user message

[zhanghanduo]

Problem

With system_addendum_per_call_role="user", each Anthropic request ends [assistant tool_use] [user tool_result…] [user addendum]. Anthropic accepts it; translating gateways do not reliably. In two Forge runs on deepseek-flash behind llm-hub (Anthropic dialect, 195 episodes), all 6 tool-pairing 400s were apodexharness requests of that shape:

• An assistant message with 'tool_calls' must be followed by tool messages responding to each 'tool_call_id'
• messages.1: tool_use ids were found without tool_result blocks immediately after

claude-code on the same model and dialect sends one user message per turn and had none. Each 400 sent ApodexHarness's loop to the clean-context forced final, and the episodes' answers were lost (3 of them on questions the other harnesses mostly solved).

#52 already folds parallel tool results into one message (unreleased since 0.12.2, so ApodexHarness's pinned image still splits them). This finishes the job for the addendum.

Change

_fold_transient_tail: after _add_prompt_cache has placed the rolling breakpoint, append trailing transient user text to the preceding user message as text blocks. Only user→user; anything else is left alone.

• The breakpoint stays on the last persistent block (the last tool_result), so the cached prefix through that block is byte-identical turn to turn. test_anthropic_rolling_breakpoint_skips_transient_addendum now compares through that block.
• test_anthropic_parallel_tool_results_share_one_user_message updated to the folded shape; new test for the plain-user-turn case.

Also: the runaway-retry reminder is per-call

A reasoning-runaway retry (_call.py) appended its reminder as a plain user message after the transient addendum, so the addendum was no longer the tail and nothing was folded: the retry went out as [tool_result] [addendum] [reminder]. Seen in the patched Forge rerun (r-20261001-180502-801e): 8 of 222 apodexharness requests, all retries. The reminder is never stored in history, so it is now marked transient too; both fold into the tool-result message and the breakpoint stays on the last tool_result.

Verification

• pytest tests: 1617 passed, 2 skipped; ruff clean. New: test_anthropic_folds_every_trailing_per_call_message, test_retry_reminder_is_per_call_so_providers_can_fold_it.
• Forge rerun with this branch's anthropic.py (before the reminder fix), apodexharness on the same 24 HLE questions: tool-pairing 400s 4 → 0, stopped_by=llm_error 3 → 0, timeouts 4 → 1, correct 7/24 → 11/24 (other four harnesses 10–12), billed tokens unchanged (3.53M → 3.52M).
• Not reproduced after the fact: replaying the three failing requests 10× each, as recorded and folded, returned 200 every time (the offending backend is intermittent). So this is the canonical shape, not a measured fix.

Release

ApodexHarness pins apodex-agent-core==0.12.2; #52 and this need a release (0.12.3) and a pin bump to reach the harness image.

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