feat(test): use new cmdlets in test VM setup and add guest agent tests

prepare-test-vm.sh now uses module cmdlets for 7 of 8 steps:
- Set-PveVmConfig -AdditionalConfig replaces SSH qm set (8 config keys)
- Test-PveVmGuestAgent replaces SSH qm agent ping
- Only SSH remaining: pvesm set, SCP snippet, qm importdisk

New integration tests for guest agent cmdlets:
- Test-PveVmGuestAgent: verify agent responds
- Get-PveVmGuestNetwork: verify interfaces with IPv4 addresses
- Invoke-PveVmGuestExec: run hostname command, verify exit code + output

Also force Node.js 24 for delete-package-versions (no v6 available yet).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.github/workflows/integration-tests.yml

@@ -95,6 +95,8 @@ jobs:
     if: always()
     runs-on: ubuntu-latest
     timeout-minutes: 5
+    env:
+      FORCE_JAVASCRIPT_ACTIONS_TO_NODE24: true
     steps:
       - name: Delete old container image versions
         uses: actions/delete-package-versions@v5