Handler validation allows objects without a real handle method

the `CommandBus::register()` uses `is_callable([$handler, 'handle'])` to validate that a handler has a public `handle()` method. However, is_callable() returns `true` for any object implementing `__call()`, even if no concrete `handle()` 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(...);
}
```