test: run CI on Windows and macOS

[kilisamemarisaaa]

What

Extend the existing test job so the suite runs on Windows and macOS alongside Ubuntu, following the cross-platform CI pattern already merged in expressjs/multer#1464 and proposed in expressjs/body-parser#759, expressjs/morgan#379, expressjs/compression#283 and expressjs/cookie-parser#171.

Changes

• strategy.matrix.os: ubuntu-latest + windows-latest + macos-latest (Node 18 and current * are installable on all three runners).
• fail-fast: false, so one platform failing does not cancel the others.
• Job and Codecov step names now carry the OS so the six legs are distinguishable.
• Before checkout, Windows jobs disable Git's automatic CRLF conversion so the formatter sees the repository's LF content.
• Windows jobs are required: the measured failure was deterministic checkout configuration, not a product or test failure.

Verification

• GitHub Actions baseline at 3e99a9b: Ubuntu and macOS passed; both Windows jobs reached npm test and failed because Prettier saw the same six CRLF-converted files.
• Local Windows CRLF checkout: npm test reproduced the same six-file Prettier failure.
• Independent Windows LF checkout with this workflow diff: npm test passed formatting, TypeScript checks, and all 47 Vitest tests.
• Workflow YAML structure assertion, npx prettier --check .github/workflows/ci.yml, and git diff --check all pass.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (e4f4fbf) to head (3e99a9b).

Additional details and impacted files

@@ Coverage Diff @@## master #105 +/- ##
=========================================
Coverage 100.00% 100.00% =========================================
Files 1 1 Lines 64 64 Branches 31 31 =========================================
Hits 64 64 

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.