test: add helper unit tests

[sonukapoor]

Summary

• add unit tests for CLI argument parsing in src/cli/args.ts
• add unit tests for severity, version, and file helper behavior in src/osv/severity.ts, src/utils/version.ts, and src/utils/file.ts
• cover default parsing, flag/value handling, severity normalization, version parsing, and file discovery helpers with deterministic tests

Testing

• npm test
• npm run build

Closes #57