test: add harness and some examples (#2941)

* test: add harness and some examples

* feat: tests ran on pr

.github/workflows/backend-tests.yaml

@@ -0,0 +1,46 @@
+name: Backend Tests
+
+on:
+  pull_request:
+    types: [opened, synchronize, reopened]
+
+permissions:
+  contents: read
+  pull-requests: write
+
+jobs:
+  test:
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v4
+
+      - name: Set up Node.js
+        uses: actions/setup-node@v4
+        with:
+          node-version: '24'
+          cache: 'npm'
+
+      - name: Install dependencies
+        run: npm ci
+
+      - name: Run backend tests with coverage
+        run: npm run test:backend -- --coverage
+        env:
+          CI: 'true'
+
+      - name: Report coverage on PR
+        if: always()
+        uses: davelosert/vitest-coverage-report-action@v2
+        with:
+          json-summary-path: src/backend/coverage/coverage-summary.json
+          json-final-path: src/backend/coverage/coverage-final.json
+
+      - name: Upload coverage artifact
+        if: always()
+        uses: actions/upload-artifact@v4
+        with:
+          name: backend-coverage
+          path: src/backend/coverage/
+          retention-days: 14