ci: update GitHub Actions to v6 (Node.js 24 support)

[clutz-bot]

Updates all GitHub Actions to v6 to resolve Node.js 20 deprecation warnings:

• actions/checkout v4 → v6
• actions/setup-node v4 → v6
• pnpm/action-setup v4 → v6

Node.js 20 actions will be forced to run with Node.js 24 starting June 2, 2026, and Node.js 20 will be removed from runners on September 16, 2026.

[changeset-bot]

⚠️ No Changeset found

Latest commit: 16c30a9

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.

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[clutz-bot]

The CI failure is a pre-existing flaky test — proxy-agent keepAlive: true test timing out on Windows (Node 20). This same test also fails intermittently on main. Not related to the action version updates in this PR. Re-triggered the failed job.