feat(ci): containerize integration tests and add Alpine guest-agent VM

Container infrastructure:
- Dockerfile with PowerShell, Pester, Terraform, sshpass,
  libguestfs-tools, and proxmox-auto-install-assistant
- build-test-container.yml workflow to build/push image to GHCR
- Image tagged ghcr.io/<repo>/integration-test:latest

Workflow restructure:
- New build job on GitHub-hosted ubuntu-latest (dotnet publish)
- Integration job runs inside Docker container on self-hosted runner
- Module artifact downloaded from build job (no .NET SDK in container)
- Host volumes mounted for PVE ISOs and cached images

Alpine guest-agent VM:
- build-test-image.sh: Downloads Alpine cloud image, installs
  qemu-guest-agent via virt-customize, caches on runner
- prepare-test-vm.sh: Deploys image to nested PVE via SCP + qm
  importdisk, configures cloud-init, waits for guest agent

Test fixes:
- Get-PveTask: use .Upid property instead of PveTask object ToString()

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

.github/workflows/build-test-container.yml

@@ -0,0 +1,34 @@
+name: Build Integration Test Container
+
+on:
+  push:
+    branches: [ main ]
+    paths:
+      - 'tests/infrastructure/Dockerfile'
+      - '.github/workflows/build-test-container.yml'
+  workflow_dispatch:
+
+permissions:
+  packages: write
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    timeout-minutes: 20
+    steps:
+      - uses: actions/checkout@v5
+
+      - name: Log in to GHCR
+        uses: docker/login-action@v3
+        with:
+          registry: ghcr.io
+          username: ${{ github.actor }}
+          password: ${{ secrets.GITHUB_TOKEN }}
+
+      - name: Build and push
+        uses: docker/build-push-action@v6
+        with:
+          context: tests/infrastructure
+          file: tests/infrastructure/Dockerfile
+          push: true
+          tags: ghcr.io/${{ github.repository }}/integration-test:latest