* 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.
