Add custom 404 and 500 error pages via ExceptionListener

- Create ExceptionListener that renders custom error templates in prod
- Skip in dev environment to keep Symfony debug pages
- 404: neo-brutalist page with large "404" text and return link
- 500: generic server error page with status code display
- Add 4 unit tests (dev skip, 404 render, 500 render, other HTTP codes)
- Register kernel.environment parameter for ExceptionListener

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

tests/EventListener/ExceptionListenerTest.php

@@ -0,0 +1,86 @@
+<?php
+
+namespace App\Tests\EventListener;
+
+use App\EventListener\ExceptionListener;
+use PHPUnit\Framework\TestCase;
+use Symfony\Component\HttpFoundation\Request;
+use Symfony\Component\HttpKernel\Event\ExceptionEvent;
+use Symfony\Component\HttpKernel\Exception\NotFoundHttpException;
+use Symfony\Component\HttpKernel\HttpKernelInterface;
+use Twig\Environment;
+
+class ExceptionListenerTest extends TestCase
+{
+    public function testDoesNothingInDevEnvironment(): void
+    {
+        $twig = $this->createMock(Environment::class);
+        $twig->expects(self::never())->method('render');
+
+        $listener = new ExceptionListener($twig, 'dev');
+        $event = $this->createEvent(new \RuntimeException('test'));
+
+        $listener($event);
+
+        self::assertNull($event->getResponse());
+    }
+
+    public function testRenders404Template(): void
+    {
+        $twig = $this->createMock(Environment::class);
+        $twig->expects(self::once())
+            ->method('render')
+            ->with('error/404.html.twig', ['status_code' => 404])
+            ->willReturn('<h1>404</h1>');
+
+        $listener = new ExceptionListener($twig, 'prod');
+        $event = $this->createEvent(new NotFoundHttpException('Not found'));
+
+        $listener($event);
+
+        self::assertNotNull($event->getResponse());
+        self::assertSame(404, $event->getResponse()->getStatusCode());
+        self::assertSame('<h1>404</h1>', $event->getResponse()->getContent());
+    }
+
+    public function testRenders500TemplateForGenericException(): void
+    {
+        $twig = $this->createMock(Environment::class);
+        $twig->expects(self::once())
+            ->method('render')
+            ->with('error/500.html.twig', ['status_code' => 500])
+            ->willReturn('<h1>500</h1>');
+
+        $listener = new ExceptionListener($twig, 'prod');
+        $event = $this->createEvent(new \RuntimeException('Server error'));
+
+        $listener($event);
+
+        self::assertNotNull($event->getResponse());
+        self::assertSame(500, $event->getResponse()->getStatusCode());
+    }
+
+    public function testRenders500TemplateForOtherHttpCodes(): void
+    {
+        $twig = $this->createMock(Environment::class);
+        $twig->expects(self::once())
+            ->method('render')
+            ->with('error/500.html.twig', ['status_code' => 403])
+            ->willReturn('<h1>403</h1>');
+
+        $listener = new ExceptionListener($twig, 'prod');
+        $event = $this->createEvent(new \Symfony\Component\HttpKernel\Exception\AccessDeniedHttpException('Forbidden'));
+
+        $listener($event);
+
+        self::assertNotNull($event->getResponse());
+        self::assertSame(403, $event->getResponse()->getStatusCode());
+    }
+
+    private function createEvent(\Throwable $exception): ExceptionEvent
+    {
+        $kernel = $this->createMock(HttpKernelInterface::class);
+
+        return new ExceptionEvent($kernel, new Request(), HttpKernelInterface::MAIN_REQUEST, $exception);
+    }
+}