fix: surface test-stdout.txt in verifier exception for dep_install classification (#540)

[ElegantLin]

Summary

• Root cause: RewardFileNotFoundError in verifier.py only included file paths in its message, never pip's actual error output. The classify_verifier_error function had dep_install markers ("no solution found", etc.) but could never match them because they lived in test-stdout.txt, not the exception.
• Fix: When raising RewardFileNotFoundError, tail the last 30 lines of test-stdout.txt into the exception message. This lets classify_verifier_error see pip's output and correctly return verifier_dep_install.
• No changes to scoring.py — the classifier logic and markers were already correct (added in prior PRs); only the wiring was broken.

Test plan

• 4 new parametrized cases in test_classify_verifier_error covering all dep_install markers in exception messages
• test_dep_install_takes_precedence_over_generic_crash — precedence test
• TestRewardFileNotFoundSurfacesStdout — 4 end-to-end wiring tests: stdout tail present, classifier fires, missing file graceful, non-dep-install stays verifier_failure
• All 96 verifier+scoring tests pass
• ruff clean

Closes #540

🤖 Generated with Claude Code

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[devin-ai-integration]

Devin Review found 1 potential issue.

View 3 additional findings in Devin Review.

[devin-ai-integration]

🟡 Hardcoded "30" in error message drifts from _TAIL_LINES constant

The error message at src/benchflow/task/verifier.py:317 uses a hardcoded string "last 30 lines" instead of referencing the _TAIL_LINES constant defined at src/benchflow/task/verifier.py:63. If _TAIL_LINES is ever changed (e.g. to 50), _tail_file will return a different number of lines but the error message will still claim "last 30 lines", misleading anyone triaging verifier failures.

Suggested change

	msg += f"\n--- test-stdout.txt (last 30 lines) ---\n{stdout_tail}"
	msg += f"\n--- test-stdout.txt (last {_TAIL_LINES} lines) ---\n{stdout_tail}"

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[bingran-you]

Thermo-nuclear review verdict: request changes.

The dependency-install classification improvement is useful, and the real Claude ACP smoke passed, but the current implementation surfaces raw verifier stdout through RewardFileNotFoundError / verifier_error. That widens the secret-leak blast radius because test-stdout.txt can contain env dumps, install URLs, tokens, or other untrusted subprocess output that later lands in summaries/dashboards.

Please redact before appending stdout tails, or keep raw stdout out of result metadata and classify from a private diagnostic path instead.

Additional maintainability issues:

• _tail_file() reads/splits the whole file to return 30 lines. Please stream-tail it, e.g. with collections.deque(f, maxlen=n).
• tests/test_verify.py grows past 1k lines. Please move these regression tests to a focused verifier-output module.
• Regression test docstrings should name the reviewed PR/fix explicitly, e.g. PR #572 / #540, per repo convention.

Verification notes:

• Branch-head checks passed: uv sync, full pytest, ty, ruff.
• Current-merge pytest and ty passed.
• Current-merge ruff failed only on existing main baseline UP041 in src/benchflow/sandbox/docker.py:408, not this PR.
• Real claude-agent-acp Docker smoke passed: reward 1.0, trajectory_source=acp, 5 ACP events, 1 tool call, usage provider_response, total tokens 64349, llm_trajectory.jsonl present.