feat: add Notification module tests and scaffolding (framework#1524)

[goravel-coder]

Summary

• Wire the Notification module into the example app: facades.Notification() facade, notification.ServiceProvider, example mail/database/queued notifications, and a notifiable User via RouteNotificationFor
• Add a notifications table migration so database-channel notifications persist alongside the example's other tables
• Add a 16-test feature suite covering send flows, custom channels, should-send/after-sending hooks, mail delivery, and artisan commands — mail sends are distinguishable because the OrderShipped subject embeds the order ID and mail tests share a mailRecipient() helper

Why

Adds full user-facing test coverage for the Notification module shipped with the pinned framework (goravel/framework#1524). The example app now wires facades.Notification() and notification.ServiceProvider, ships three ready-to-use example notifications (mail, database, and queued), makes User notifiable via RouteNotificationFor, and persists database-channel notifications in a new notifications migration.

The 16-test feature suite asserts user-visible outcomes — persisted rows in the notifications table, successful sends, channel skipping, and command-generated files. The mail tests were refined so each send is identifiable: OrderShipped.ToMail now embeds the order ID in the subject (matching the HTML body), and both mail tests resolve their recipient through a shared mailRecipient() helper that skips when MAIL_HOST is unset.

// app/notifications/order_shipped.go
func (r *OrderShipped) ToMail(notifiable notification.Notifiable) notification.MailMessage {
	return mail.NewMessage().
		Subject("Order " + r.OrderID + " has shipped").
		Html("<p>Order " + r.OrderID + " has shipped.</p>").
		Build()
}

// Send from a controller
if err := facades.Notification().Send(user, notifications.NewOrderShipped(orderID)); err != nil {
	return ctx.Response().Json(http.StatusInternalServerError, http.Json{"error": err.Error()})
}