fix: switch from xdebug to pcov for code coverage (#101)

* fix: remove xdebug-specific settings from phpunit.xml and composer.json

Remove `<ini name="xdebug.mode" value="coverage"/>` from phpunit.xml and
`XDEBUG_MODE=coverage` prefix from composer scripts. These fail when xdebug
is not installed and are unnecessary when using PCOV for code coverage.

* ci: switch from xdebug to pcov for code coverage

Both test matrix and coverage jobs now use `coverage: pcov`, matching
the existing test-coverage.yaml workflow. PCOV is faster for
coverage-only use and avoids the xdebug dependency conflict.

* build: replace xdebug with pcov in Docker setup

Switch Docker development environment from xdebug to pcov for code
coverage and remove the XDEBUG_MODE environment variable from
docker-compose.yml. Standardizes on pcov across all environments.

docker/docker-compose.yml

@@ -11,7 +11,6 @@ services:
     working_dir: /app
     environment:
       - COMPOSER_HOME=/home/developer/.composer
-      - XDEBUG_MODE=coverage
     stdin_open: true
     tty: true
     command: tail -f /dev/null