Add mutation tests to GitHub workflows

- Updated `php.yml` and `downgrade-php74.yml` to include mutation testing steps using `composer run-script mutate`.

Author: georgedeploy

.github/workflows/downgrade-php74.yml

@@ -61,6 +61,9 @@ jobs:
       - name: Run test coverage
         run: composer run-script coverage
 
+      - name: Run mutation tests
+        run: composer run-script mutate
+
   downgrade-and-tag:
     name: Downgrade to PHP 7.4 and Tag v1.x
     needs: verify-php82

.github/workflows/php.yml

@@ -51,4 +51,7 @@ jobs:
       run: composer run-script type
 
     - name: Run test coverage
-      run: composer run-script coverage
+      run: composer run-script coverage
+
+    - name: Run mutation tests
+      run: composer run-script mutate