Enable Xdebug coverage mode explicitly

Added XDEBUG_MODE=coverage environment variable to ensure PHPUnit uses Xdebug for coverage generation. Without this, Xdebug may not activate coverage even though it's installed.

Author: aristath

.github/workflows/code-coverage.yml

@@ -65,7 +65,7 @@ jobs:
 
       - name: Generate code coverage report for current branch
         run: |
-          vendor/bin/phpunit --dont-report-useless-tests --coverage-clover=${{ github.workspace }}/coverage.xml --coverage-text | tee coverage-output.txt
+          XDEBUG_MODE=coverage vendor/bin/phpunit --dont-report-useless-tests --coverage-clover=${{ github.workspace }}/coverage.xml --coverage-text | tee coverage-output.txt
         continue-on-error: false
 
       - name: Upload coverage to Codecov