ci: switch from xdebug to pcov for code coverage

[marcstraube]

Problem

GitLab CI uses xdebug for code coverage. xdebug is a full debugger with significant overhead for coverage-only use cases.

Solution

Switch to pcov — a dedicated, lightweight coverage driver:

.php-setup: &php-setup
  before_script:
    - apt-get update -yqq
    - apt-get install -yqq git unzip libzip-dev
    - docker-php-ext-install zip
    - pecl install pcov && docker-php-ext-enable pcov
    - curl -sS https://getcomposer.org/installer | php -- --install-dir=/usr/local/bin --filename=composer
    - composer install --prefer-dist --no-progress --no-interaction

Also remove XDEBUG_MODE: coverage from variables.

Benefits:

• Significantly faster coverage collection
• Lower memory usage
• No debugger overhead

Reference

• zappzarapp/audit-logger .gitlab-ci.yml