test: Rust CLI contract 회귀 검증 강화

[JeremyDev87]

요약

• Rust CLI contract 테스트에 help/version alias와 no-command help 경계를 추가했습니다.
• scan --json 출력은 raw serde_json::Value 기반 normalized oracle 비교로 고정해 unknown field drift를 놓치지 않게 했습니다.
• invalid argv 케이스에 exit code 1 검증과 --top missing-value 회귀 테스트를 추가했습니다.

검증

• cargo test -p legolas-cli --test cli_contract
• cargo test --workspace

[JeremyDev87]

Independent fresh-session review result

• Verdict: APPROVE
• Findings: none
• Rationale: The PR only expands Rust CLI contract coverage and shared JSON normalization helpers, and the affected cli_contract and text_report_parity suites passed without surfacing a regression.

Note: Official approve/request-changes could not be submitted because the PR is self-authored, so this was posted as a PR comment.