chore: use local XSD for PHPUnit configuration

Reference the vendored phpunit.xsd instead of the remote schema URL
to avoid network dependencies during IDE validation.

References:
- infection/infection#2303
- sebastianbergmann/phpunit#6289

Author: claude

phpunit.xml.dist

@@ -1,5 +1,5 @@
 <?xml version="1.0" encoding="UTF-8"?>
-<phpunit xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" backupGlobals="false" bootstrap="vendor/autoload.php" colors="true" processIsolation="false" stopOnFailure="false" xsi:noNamespaceSchemaLocation="https://schema.phpunit.de/12.5/phpunit.xsd" cacheDirectory=".phpunit.cache" backupStaticProperties="false" failOnWarning="true">
+<phpunit xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" backupGlobals="false" bootstrap="vendor/autoload.php" colors="true" processIsolation="false" stopOnFailure="false" xsi:noNamespaceSchemaLocation="vendor/phpunit/phpunit/phpunit.xsd" cacheDirectory=".phpunit.cache" backupStaticProperties="false" failOnWarning="true">
   <logging>
     <junit outputFile="build/logs/results.xml"/>
   </logging>