Add bounded final-answer acceptance gate in regular runner

[mmprotest]

Motivation

• Prevent premature finalization by the regular runner when the assistant claims a concrete edit, when edits were made but not verified, or when the last verification failed and no corrective action followed.
• Keep the change small, conservative, one-shot, and non-benchmark-specific while exempting analysis/plan-only requests.

Description

• Added a centralized helper final_answer_block_reason(...) in villani_code/state.py that returns a one-off blocking nudge reason (or None) for three conservative cases: concrete edit claimed with no edits, changed files with a known verification command but no verification since the last edit, and failed verification with no corrective follow-up; it respects plan/analysis-only instructions and uses a per-run nudge_state to avoid repeated blocking.
• Wired that helper into the runner completion path in villani_code/state.py, tracking minimal per-run bookkeeping (last_edit_turn, last_verification_turn, last_failure_followup_turn, finalization_nudge_state) and updating those markers when Write/Patch/Bash/inspection tools run so the gate has conservative inputs.
• Improved verification feedback in villani_code/state_runtime.py by extracting focused visible_failure_lines (assertions, traceback/file refs, pytest E lines, and keywords like expected/actual) and appending a generic guidance sentence: "Treat visible test assertions as authoritative evidence. Do not guess replacement values when the expected value appears in the failure output."
• Added unit tests tests/test_final_answer_gate.py that exercise the helper for the required behaviors and one-shot semantics.

Testing

• Ran the new unit tests with pytest -q tests/test_final_answer_gate.py tests/test_state_runtime.py::test_verification_detail_event_keeps_raw_trace, and the test run passed (8 passed).
• The added tests directly exercise final_answer_block_reason(...) for: concrete edit claim without edits (blocks once), vague suggestions (no block), plan-only exemption (no block), changed files without verification when verification is known (blocks), changed files without a known verification command (no block), failed verification with and without corrective follow-up (blocks/does not block respectively).

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Codex Task