Add functional test for grabContainer()

Cover grabContainer(), added in codeception/module-symfony: assert it returns the
container for the test environment, and that it is the test container rather than
the kernel's own. The last two assertions are the point of the method: the test
container resolves a private service that the kernel container cannot see, which
is what made the widespread grabService('kernel')->getContainer() workaround
subtly wrong.

No app fixtures are needed: EntityManagerInterface is registered as a private
autowiring alias by doctrine-bundle on every branch.

The test is identical on every branch.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01H9UKS58pxkiV3ovVP9J4tB

Author: TavoNiievez

tests/Functional/ServicesCest.php

@@ -6,10 +6,24 @@
 
 use App\Service\Greeting;
 use App\Tests\Support\FunctionalTester;
+use Doctrine\ORM\EntityManagerInterface;
 use Symfony\Bundle\SecurityBundle\Security;
+use Symfony\Component\DependencyInjection\ContainerInterface;
 
 final class ServicesCest
 {
+    public function grabContainer(FunctionalTester $I): void
+    {
+        $container = $I->grabContainer();
+
+        $I->assertInstanceOf(ContainerInterface::class, $container);
+        $I->assertSame('test', $container->getParameter('kernel.environment'));
+
+        // Unlike the kernel's container, the test container also exposes private services.
+        $I->assertTrue($container->has(EntityManagerInterface::class));
+        $I->assertFalse($I->grabService('kernel')->getContainer()->has(EntityManagerInterface::class));
+    }
+
     public function grabService(FunctionalTester $I)
     {
         $security = $I->grabService('security.helper');