ci: add mutation testing workflow with Infection

- Add Infection to composer.json require-dev dependencies
- Create GitHub workflow for mutation testing
- Configure workflow to fail if MSI is below 87%
- Set threads to max for optimal performance
- Run on push to main and pull requests

Author: claude

.github/workflows/mutation-testing.yml

@@ -0,0 +1,44 @@
+name: Mutation Testing
+
+on:
+  push:
+    branches:
+      - main
+  pull_request:
+
+permissions:
+  contents: read
+
+jobs:
+  mutation-testing:
+    name: Mutation Testing
+    runs-on: ubuntu-24.04
+
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@8e8c483db84b4bee98b60c0593521ed34d9990e8 # v6
+
+      - name: Setup PHP
+        uses: shivammathur/setup-php@44454db4f0199b8b9685a5d763dc37cbf79108e1 # v2
+        with:
+          php-version: '8.2'
+          extensions: json
+          coverage: xdebug
+          tools: composer:v2
+
+      - name: Get composer cache directory
+        id: composer-cache
+        run: echo "dir=$(composer config cache-files-dir)" >> $GITHUB_OUTPUT
+
+      - name: Cache dependencies
+        uses: actions/cache@9255dc7a253b0ccc959486e2bca901246202afeb # v5
+        with:
+          path: ${{ steps.composer-cache.outputs.dir }}
+          key: ${{ runner.os }}-composer-${{ hashFiles('**/composer.json') }}
+          restore-keys: ${{ runner.os }}-composer-
+
+      - name: Install dependencies
+        run: composer update -n --prefer-dist
+
+      - name: Run mutation testing
+        run: vendor/bin/infection --threads=max --min-msi=87 --min-covered-msi=87

composer.json

@@ -26,7 +26,8 @@
         "phpunit/phpunit": "^11||^12",
         "guzzlehttp/guzzle": "^7.0",
         "humbug/box": "^4.6",
-        "phpstan/phpstan": "^2.0"
+        "phpstan/phpstan": "^2.0",
+        "infection/infection": "^0.30"
     },
     "autoload": {
         "psr-4": {