Upgrade to PHP 8.5 and apply Rector cleanup

[kayjoosten]

Goal

Upgrade PHP runtime to 8.5, update Composer dependencies, and apply Rector cleanup.

What needs to be done

• Bump composer.json PHP constraint to ^8.5, run composer update
• Update Dockerfile(s)/docker-compose and CI to PHP 8.5
• Add rector/rector: ^2.4, add rector.php:

  return RectorConfig::configure()
      ->withPaths([__DIR__.'/bin', __DIR__.'/config', __DIR__.'/src'])
      ->withPhpSets()
      ->withComposerBased(symfony: true)
      ->withAttributesSets(symfony: true)
      ->withSkip([
          \Rector\Php84\Rector\MethodCall\NewMethodCallWithoutParenthesesRector::class,
          \Rector\Php84\Rector\Class_\DeprecatedAnnotationToDeprecatedAttributeRector::class,
      ]);

• Apply rector, fix what it can't (implicit nullable params, __sleep/__wakeup, removed functions like xml_parser_free())
• Full test suite + static analysis green on PHP 8.5