ci: add mutation testing

.github/workflows/testing.yml

@@ -74,3 +74,61 @@ jobs:
           files: .build/phpunit/logs/clover.xml
           token: ${{ secrets.CODECOV_TOKEN }}
           verbose: true
+
+  mutation-testing:
+    timeout-minutes: 16
+    runs-on: ${{ matrix.os }}
+    concurrency:
+      cancel-in-progress: true
+      group: mutation-testing-${{ github.workflow }}-${{ github.event.pull_request.number || github.ref }}
+    strategy:
+      fail-fast: true
+      matrix:
+        os:
+          - ubuntu-latest
+        php-version:
+          - '8.2'
+        dependencies:
+          - locked
+    steps:
+      - name: 📦 Check out the codebase
+        uses: actions/checkout@v4.1.5
+
+      - name: 🛠️ Setup PHP
+        uses: shivammathur/setup-php@2.30.4
+        with:
+          php-version: ${{ matrix.php-version }}
+          extensions: none, ctype, dom, json, mbstring, simplexml, tokenizer, xml, xmlwriter, pdo, curl, fileinfo, pdo_mysql, pdo_sqlite
+          ini-values: error_reporting=E_ALL
+          coverage: xdebug
+
+      - name: 🛠️ Setup problem matchers
+        run: |
+          echo "::add-matcher::${{ runner.tool_cache }}/php.json"
+          echo "::add-matcher::${{ runner.tool_cache }}/phpunit.json"
+
+      - name: 🤖 Validate composer.json and composer.lock
+        run: composer validate --ansi --strict
+
+      - name: 🔍 Get composer cache directory
+        uses: wayofdev/gh-actions/actions/composer/get-cache-directory@v3.1.0
+
+      - name: ♻️ Restore cached dependencies installed with composer
+        uses: actions/cache@v4.0.2
+        with:
+          path: ${{ env.COMPOSER_CACHE_DIR }}
+          key: php-${{ matrix.php-version }}-composer-${{ matrix.dependencies }}-${{ hashFiles('composer.lock') }}
+          restore-keys: php-${{ matrix.php-version }}-composer-${{ matrix.dependencies }}-
+
+      - name: 📥 Install "${{ matrix.dependencies }}" dependencies
+        uses: wayofdev/gh-actions/actions/composer/install@v3.1.0
+        with:
+          dependencies: ${{ matrix.dependencies }}
+
+      - name: 🧪 Run mutation testing using Xdebug and infection/infection
+        env:
+          STRYKER_DASHBOARD_API_KEY: ${{ secrets.STRYKER_DASHBOARD_API_KEY }}
+          XDEBUG_MODE: coverage
+          DB_CONNECTION: sqlite
+          DB_DATABASE: ':memory:'
+        run: composer infect:ci