Add utility coverage tests

[eutv2022]

Summary

• Add focused Vitest coverage for arrays, dates, money, and string helpers.
• Cover edge cases such as invalid chunk sizes, weekend detection, fee guards, remainder splitting, truncation, title casing, and HTML escaping.

Verification

• npm test (4 files, 19 tests)
• npx vitest run --coverage --coverage.reporter=text-summary --coverage.include=src/**/*.ts --coverage.exclude=src/**/*.test.ts
  • Statements: 100% (80/80)
  • Branches: 97.22% (35/36)
  • Functions: 100% (15/15)
  • Lines: 100% (80/80)
• git diff --check

This targets the TaskBounty coverage uplift task: https://www.task-bounty.com/task/coverage-uplift-starter-eliottreich-taskbounty-dog-4ug8k7