ci: add Windows to the test matrix (verification phase)

[shulaoda]

Warning

Not for merge as-is. This PR is a verification harness for the Windows support effort: it runs the full test matrix on Windows for every PR so we can see and fix Windows failures on follow-up PRs. Once Windows is green, the matrix will be flipped to main-only (so PRs aren't slowed) and only then will the Windows matrix be merged.

Summary

• Add windows-latest to the os matrix of: test-unit, test-integration, test-integration-merge, create-next-app, e2e.
• Name CI artifacts per-OS (blob-report-<os>-<shard>, coverage-html-<os>, playwright-report-<project>-<os>) so Windows and Linux runs don't collide.
• Guard the Linux-only WebKit system-dependency steps (apt/sudo) with runner.os == 'Linux'.
• Coverage merge stays Linux-only (it's OS-independent).

Why run on PRs for now

The goal of this PR is to make Windows failures visible so they can be fixed incrementally. Running on PRs (not just main) lets each follow-up fix PR show its Windows status directly. The CI aggregate (CI) will be red until the ~known Windows failures are resolved — that is expected during this phase.

Switching to main-only before merge

Each test job's os: is currently the static [ubuntu-latest, windows-latest]. To make Windows main-only (Linux-only on PRs) before merging, replace each with the gated form documented at the top of the jobs: block:

os: ${{ github.event_name == 'push' && fromJSON('["ubuntu-latest", "windows-latest"]') || fromJSON('["ubuntu-latest"]') }}

Not included

• No tests or source are changed here — this is CI config only. Windows fixes land in separate PRs.
• E2E projects (including Cloudflare/wrangler ones) are not excluded on Windows; failures there will be triaged on follow-up PRs rather than hidden.

Notes / to verify before merge

• Confirm the required status check on main is the aggregate CI job (not individual matrix legs). Adding ${{ matrix.os }} to leg names changes their display names; if branch protection required specific leg names, those would need updating. The existing shard-parameterized leg names suggest the gate is already the CI aggregate.

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commit: f22e2ad