Add PHP CS Fixer and PHPStan max to CI; remove abandoned GoutteFactory

- Add friendsofphp/php-cs-fixer ^3.75 and phpstan/phpstan ^2.0 to
  require-dev; add .php-cs-fixer.dist.php (@Symfony ruleset with
  phpdoc_to_comment ignored_tags) and phpstan.neon (level max,
  treatPhpDocTypesAsCertain: false)
- Run CS Fixer and PHPStan in every CI matrix job alongside tests
- Add composer scripts: cs, cs-check, phpstan
- Add .php-cs-fixer.cache to .gitignore
- Fix all PHPStan max violations across src/: add return/param types,
  narrow mixed config values with is_string()/is_array() guards,
  use TaggedNodeInterface for scenario tag access in SessionsListener
- Remove GoutteFactory and its spec — the goutte driver and its
  underlying client library are abandoned

Co-Authored-By: Claude Sonnet 4.6 (1M context) <noreply@anthropic.com>

spec/Behat/MinkExtension/ServiceContainer/Driver/BrowserStackFactorySpec.php

@@ -3,21 +3,20 @@
 namespace spec\Behat\MinkExtension\ServiceContainer\Driver;
 
 use PhpSpec\ObjectBehavior;
-use Prophecy\Argument;
 
 class BrowserStackFactorySpec extends ObjectBehavior
 {
-    function it_is_a_driver_factory()
+    public function it_is_a_driver_factory()
     {
         $this->shouldHaveType('Behat\MinkExtension\ServiceContainer\Driver\DriverFactory');
     }
 
-    function it_is_named_browser_stack()
+    public function it_is_named_browser_stack()
     {
         $this->getDriverName()->shouldReturn('browser_stack');
     }
 
-    function it_supports_javascript()
+    public function it_supports_javascript()
     {
         $this->supportsJavascript()->shouldBe(true);
     }