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.

.github/workflows/ci.yml

@@ -33,7 +33,7 @@ jobs:
           php-version: ${{ matrix.php-version }}
           extensions: mbstring, xml, ctype, iconv, intl, json
           tools: composer:v2
-          coverage: xdebug
+          coverage: pcov
 
       - name: Get composer cache directory
         id: composer-cache
@@ -80,7 +80,7 @@ jobs:
           php-version-file: '.php-version'
           extensions: mbstring, xml, ctype, iconv, intl, json
           tools: composer:v2
-          coverage: xdebug
+          coverage: pcov
 
       - name: Install dependencies
         run: composer install --prefer-dist --no-progress