TestCaseFactory for flexible test instantiation

[butschster]

This PR introduces the TestCaseFactory, providing developers with complete control over how test case instances are created. Previously, test classes were instantiated using basic reflection, limiting support for dependency injection and complex initialization scenarios.

What's New

Three Implementation Strategies:

1. ReflectionFactory (default) - Simple reflection-based instantiation
2. ContainerFactory - PSR-11 container integration for dependency injection
3. CallableFactory - Custom factory logic via user-defined closures

---

💡 Motivation

Problem

Test classes with constructor dependencies couldn't be instantiated:

final class UserRepositoryTest 
{
    // ❌ This previously failed - no way to inject dependencies
    public function __construct(
        private DatabaseConnection $db,
        private UserRepository $repository,
    ) {}
}

Solution

Now developers can configure a custom factory:

// Example 1: Using PSR-11 Container
$container = new Container();
$container->set(DatabaseConnection::class, $dbConnection);

$app = Application::createFromConfig(
    config: $config,
    testCaseFactory: new ContainerFactory($container)
);

// Example 2: Custom Factory Logic
$app = Application::createFromConfig(
    config: $config,
    testCaseFactory: new CallableFactory(
        fn(\ReflectionClass $class) => $myFactory->create($class->getName())
    )
);