Avoid ReflectionMethod::setAccessible() deprecations on PHP 8.5+ in serializer tests

[Copilot]

PHP 8.5 deprecates ReflectionMethod::setAccessible() (no-op since 8.1), which causes the PHPUnit 8.5 job to fail on deprecation-as-failure. This change removes those deprecations while preserving test behavior on older PHP versions.

• Scope

  • Updated serializer test helpers that reflect protected methods:
    • test/HTML5/Serializer/TraverserTest.php
    • test/HTML5/Serializer/OutputRulesTest.php

• Change

  • Guarded setAccessible(true) behind a PHP version check so it is only called where still relevant (< 8.5).

• Behavior impact

  • On PHP 8.5+: no deprecated setAccessible() calls.
  • On older runtimes: existing reflection access pattern remains unchanged.

$method = $class->getMethod($name);
if (\PHP_VERSION_ID < 80500) {
    $method->setAccessible(true);
}