Add E2E snapshot tests for validate-audit.js error output paths

[coderabbitai]

Summary

Snapshot tests for advisory reporting helpers are well-designed, but end-to-end snapshot tests for validate-audit.js error output are missing despite the PR recommendation.

Problems

• No snapshot tests exist for the following error paths in validate-audit.js:
  • AJV schema validation errors (malformed audit-exceptions.json).
  • Unexpected advisory detection (reportUnexpectedAdvisories output).
  • Validator vulnerability detection errors (advisory present in audit but not in exception ledger).

Acceptance criteria

• Add snapshot tests (using Vitest toMatchInlineSnapshot or toMatchSnapshot) for validate-audit.js error paths, covering at minimum:
  • Schema validation failure output.
  • Unexpected-advisory heading and bullet-line output.
  • At least one end-to-end path that drives validate-audit.js from a controlled advisory fixture through to the final error message captured from reportingIo.

Context

Raised during review of PR #354 by @leynos.

[coderabbitai]

🔗 Related PRs

#350 - Extract audit fallback test setup [merged]

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