test: raise utility coverage to 100%

[MFei8ht]

Summary

Adds focused Vitest coverage for the tiny utility modules used by the TaskBounty coverage uplift task:

• array helpers: chunking, dedupe, and grouping
• date helpers: day distance, weekend detection, day addition, and UTC day normalization
• money helpers: formatting, platform fees, net payout, and even splits
• string helpers: slugifying edge cases, truncation, title casing, and HTML escaping

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

Verification

• npm test - 23 tests passed
• npx vitest run --coverage --coverage.reporter=text-summary --coverage.include=src/**/*.ts --coverage.exclude=src/**/*.test.ts - 100% statements / 96.96% branches / 100% functions / 100% lines

[MFei8ht]

TaskBounty traceability note:

This PR targets the current public feed item [$5] Coverage Uplift Starter - eliottreich/taskbounty-dogfood-tiny-utils.

Local verification:

• npm test: 23 tests passing
• direct coverage run: 100% statements, 96.96% branches, 100% functions, 100% lines

I also tried the TaskBounty submissions API with task id 37703bba-8af5-4068-b438-fad88c87ae40, but unauthenticated submission returns 401, so I'm leaving the task link and verification here for review.

[MFei8ht]

Small follow-up pushed in 195c906:

• changed npm run test:coverage to use JSON-escaped double quotes instead of single quotes, so the repository script reports coverage correctly on Windows too.

Re-verified locally:

• npm test: 23 tests passing
• npm run test:coverage: 100% statements, 96.96% branches, 100% functions, 100% lines

[MFei8ht]

Quick TaskBounty submission-path note: the public feed still lists this $5 Coverage Uplift task with id 37703bba-8af5-4068-b438-fad88c87ae40, but the task URL is currently returning 404 for me.

The PR is still ready from my side. Verification remains:

• npm test: 23 tests passing
• npm run test:coverage: 100% statements, 96.96% branches, 100% functions, 100% lines

If there is a preferred TaskBounty claim/submission path for this PR, I can follow it.