feat: add webhook placeholder to Test notification

Add webhook case to the Test notification's via() method
to prepare for future WebhookChannel implementation.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

app/Notifications/Test.php

@@ -36,6 +36,7 @@ public function via(object $notifiable): array
                 'telegram' => [TelegramChannel::class],
                 'slack' => [SlackChannel::class],
                 'pushover' => [PushoverChannel::class],
+                'webhook' => [], // WebhookChannel will be implemented later
                 default => [],
             };
         } else {