ci: add GitHub Actions workflow to test the action

Tests include:
- Cross-platform testing (ubuntu, macos, windows)
- Cache functionality verification
- Build and type check validation
- Dist freshness check

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: fengmk2

.github/workflows/test.yml

@@ -0,0 +1,85 @@
+name: Test
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+    branches: [main]
+
+jobs:
+  test:
+    strategy:
+      fail-fast: false
+      matrix:
+        os: [ubuntu-latest, macos-latest, windows-latest]
+    runs-on: ${{ matrix.os }}
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: actions/setup-node@v4
+        with:
+          node-version: '22'
+
+      - name: Setup Vite+
+        uses: ./
+        with:
+          version: latest
+          cache: false
+
+      - name: Verify installation
+        run: |
+          vp --version
+          vite --version
+
+  test-cache:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: actions/setup-node@v4
+        with:
+          node-version: '22'
+
+      - name: Create test project
+        run: |
+          mkdir -p test-project
+          cd test-project
+          echo '{"name":"test-project","private":true}' > package.json
+
+      - name: Setup Vite+ with cache
+        uses: ./
+        id: setup
+        with:
+          cache: true
+          cache-dependency-path: pnpm-lock.yaml
+
+      - name: Verify installation
+        run: |
+          vp --version
+          echo "Installed version: ${{ steps.setup.outputs.version }}"
+          echo "Cache hit: ${{ steps.setup.outputs.cache-hit }}"
+
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: pnpm/action-setup@v4
+
+      - uses: actions/setup-node@v4
+        with:
+          node-version: '22'
+          cache: 'pnpm'
+
+      - name: Install dependencies
+        run: pnpm install
+
+      - name: Type check
+        run: pnpm run typecheck
+
+      - name: Build
+        run: pnpm run build
+
+      - name: Verify dist is up to date
+        run: |
+          git diff --exit-code dist/ || (echo "dist/ is out of date. Run 'pnpm run build' and commit." && exit 1)