ci: run tests on windows and macos matrix

[kilisamemarisaaa]

Motivation

• Are our tests reliable? #592 (discuss) already identified the platform coverage gap: CI only runs on ubuntu-latest, and the 2026-08-27 comment in that discussion raised exactly this (platform coverage is the biggest remaining reliability gap for body-parser).
• This follows the accepted pattern from the same org: multer#1334 extended the test matrix to ubuntu-latest + windows-latest + macos-latest with cross-platform assertions, merged 2026-08-24.

Changes

• test job: add os to strategy.matrix ([ubuntu-latest, windows-latest, macos-latest]) and switch runs-on to ${{ matrix.os }}. fail-fast: false and the node-version list are unchanged.
• Coverage artifact names now include the OS (coverage-node-${{ matrix.os }}-${{ matrix.node-version }}) so the 3 OS × 10 Node matrix legs do not collide on a shared artifact name — actions/upload-artifact v4+ rejects duplicate artifact names with a 409 conflict. The coverage job is untouched: it still collects everything matching coverage-node-* and the coverage-node-* prefix is unchanged.
• lint and coverage jobs remain on ubuntu-latest.

Verification

• Local Windows baseline (this PR's windows-latest leg): npm install, npm test, npm run lint all green — 273 passing, 0 failing. No test-file changes were needed; the suite is already platform-agnostic (no path/sep/process.platform usage; the only timing-sensitive assertion in test/text.js passes on Windows).
• Full matrix (3 OS × 10 Node) has not executed yet: the ci and CodeQL workflow runs for this PR are pending maintainer approval (first-time contributor workflow). Only the local Windows baseline (Node 24: 273 passing, lint green) has been verified so far; the matrix will run once the maintainers approve the runs.