fix(generator): resolve model namespace without app/ prefix in module observer stub

[malayvuong]

Description

This PR fixes the incorrect model namespace generated in observers.

Previously, running:

php artisan module:make-observer MenuItem Core

would generate:

<?php

namespace Modules\Core\Observers;

use Modules\Core\app\Models\MenuItem; // ❌ wrong (includes "app")
use App\Observers\AbstractObserver;

final class MenuItemObserver extends AbstractObserver
{
    protected function modelClass(): string
    {
        return MenuItem::class;
    }
}

Root Cause

The command was converting the generator path (app/Models) directly into a namespace, which caused the app/ segment to leak into the namespace (Modules\Core\app\Models).

Fix

• Strip the configured app_folder prefix (app/) from the generator path before building the namespace.
• After this change, the generated observer correctly imports the model as:

use Modules\Core\Models\MenuItem; // ✅ correct

Impact

• Works with the default app/ folder.
• Also works if app_folder is customized (e.g. src/) in the module config.
• No breaking changes, only affects namespace resolution in generated classes.