Middleware validation allows objects without a real execute method

The `CommandBus::addMiddleware()` uses `is_callable([$middleware, 'execute'])` to validate that middleware has a public `execute()` method. However, `is_callable()` returns `true` for any object implementing `__call()`, even if no concrete `execute()` method exists — contradicting the exception message and documented contract.

Fix: Replace the `is_callable()` check with `method_exists()` + `ReflectionMethod::isPublic():`

```php
isPublic()) {
throw new InvalidArgumentException(...);
}
```