fix: route corrupt CatBoost scans fail closed

[mldangelo-oai]

Summary

This PR fixes a CatBoost routing fail-open discovered during the correctness audit. A .cbm file with the CatBoost CBM1 magic but a corrupt declared core section was detected as CatBoost format, but CatBoostScanner.can_handle rejected it before the scanner could report the structural failure. The aggregate scan then produced only a DEBUG unknown-format issue and exited 0.

Root Cause

CatBoostScanner.can_handle required a fully in-bounds declared core section. That made can_handle behave as a validity check instead of a routing check. Corrupt CatBoost files therefore bypassed the scanner that knows how to fail closed on CatBoost structure errors.

Changes

• Loosen CatBoost can_handle to route .cbm files with the CBM1 magic into CatBoostScanner even when the structure is corrupt.
• Mark CatBoost parse/read failures with scan_outcome=inconclusive metadata so aggregate scans return exit code 2 when no security finding is recovered.
• Add regression coverage for corrupt-magic routing, direct structure parse metadata, and aggregate exit code 2.
• Update the Unreleased changelog.

Validation

• uv run pytest tests/scanners/test_catboost_scanner.py: 10 passed
• uv run ruff format modelaudit/ packages/modelaudit-picklescan/src packages/modelaudit-picklescan/tests tests/
• uv run ruff check --fix modelaudit/ packages/modelaudit-picklescan/src packages/modelaudit-picklescan/tests tests/
• uv run mypy modelaudit/ packages/modelaudit-picklescan/src packages/modelaudit-picklescan/tests tests/: 432 source files clean
• uv run pytest -n auto -m "not slow and not integration" --maxfail=1: 2432 passed, 1100 skipped, 16 warnings
• uv run ruff check modelaudit/ packages/modelaudit-picklescan/src packages/modelaudit-picklescan/tests tests/
• uv run ruff format --check modelaudit/ packages/modelaudit-picklescan/src packages/modelaudit-picklescan/tests tests/
• git diff --check

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Repository UI

Review profile: ASSERTIVE

Plan: Pro

Run ID: b07e4e0f-f5df-4c37-b4cf-21c641f4b708

📥 Commits

Reviewing files that changed from the base of the PR and between f285a05 and 9f0383c.

📒 Files selected for processing (3)

• CHANGELOG.md
• modelaudit/scanners/catboost_scanner.py
• tests/scanners/test_catboost_scanner.py

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch mdangelo/codex/catboost-routing-audit

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Workflow run and artifacts

Performance Benchmarks

Compared 6 shared benchmarks with a regression threshold of 15%.
Status: 0 regressions, 0 improved, 6 stable, 0 new, 0 missing.
Aggregate shared-benchmark median: 684.90ms -> 696.58ms (+1.7%).

Benchmark	Target	Size	Files	Baseline	Current	Change	Status
tests/benchmarks/test_scan_benchmarks.py::test_detect_file_format_safe_pickle	safe_model.pkl	49.4 KiB	1	166.1us	173.7us	+4.6%	stable
tests/benchmarks/test_scan_benchmarks.py::test_scan_pytorch_zip	state_dict.pt	1.5 MiB	1	34.16ms	35.12ms	+2.8%	stable
tests/benchmarks/test_scan_benchmarks.py::test_scan_duplicate_directory	duplicate-corpus	840.0 KiB	81	484.25ms	492.94ms	+1.8%	stable
tests/benchmarks/test_scan_benchmarks.py::test_scan_mixed_directory	mixed-corpus	1.7 MiB	54	137.73ms	140.19ms	+1.8%	stable
tests/benchmarks/test_scan_benchmarks.py::test_scan_safe_pickle	safe_model.pkl	49.4 KiB	1	28.54ms	28.11ms	-1.5%	stable
tests/benchmarks/test_scan_benchmarks.py::test_validate_file_type_pytorch_zip	state_dict.pt	1.5 MiB	1	47.9us	48.0us	+0.2%	stable