feat(mail-test): implement email testing functionality

- Created MailTestController to handle email sending requests.
- Added SendTestMailRequest for validating email input.
- Developed MailTestService to manage email sending logic.
- Introduced TestMail Mailable for formatting test emails.
- Defined API route for sending test emails.
- Created EmailIntegrationSeeder to seed email integration settings.
- Updated DatabaseSeeder to include EmailIntegrationSeeder.
- Added MailTestControllerTest to ensure email sending functionality works as expected.

app/Domains/MailTest/routes/api.php

@@ -0,0 +1,6 @@
+<?php
+
+use App\Domains\MailTest\Controllers\MailTestController;
+use Illuminate\Support\Facades\Route;
+
+Route::post('mail-test/send', MailTestController::class);