Add test notification channel feature with AJAX support

Introduces the ability to test notification channels (email, Telegram, Discord, Slack) from the group edit page, both for new and existing channels. Adds a new testChannel method to NotificationGroupController with AJAX and form support, improves validation and error handling, and updates the UI to include test buttons and dynamic toast notifications. Also registers the new /channels/test route.

routes/web.php

@@ -85,6 +85,7 @@ $router->post('/groups/bulk-delete', [NotificationGroupController::class, 'bulkD
 $router->post('/channels/add', [NotificationGroupController::class, 'addChannel']);
 $router->get('/channels/delete', [NotificationGroupController::class, 'deleteChannel']);
 $router->get('/channels/toggle', [NotificationGroupController::class, 'toggleChannel']);
+$router->post('/channels/test', [NotificationGroupController::class, 'testChannel']);
 
 // TLD Registry
 $router->get('/tld-registry', [TldRegistryController::class, 'index']);