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/Services/MailTestService.php

@@ -0,0 +1,27 @@
+<?php
+
+namespace App\Domains\MailTest\Services;
+
+use App\Domains\MailTest\Mailables\TestMail;
+use Illuminate\Support\Facades\Mail;
+
+class MailTestService
+{
+    /**
+     * @return array<string, string>
+     */
+    public function send(string $recipient, ?string $subject = null, ?string $message = null): array
+    {
+        $subject ??= 'Prueba de correo de Shopit';
+        $message ??= 'Este es un correo de prueba enviado desde Shopit.';
+
+        Mail::to($recipient)->send(new TestMail($subject, $message));
+
+        return [
+            'message' => 'Correo de prueba enviado correctamente.',
+            'recipient' => $recipient,
+            'mailer' => (string) config('mail.default'),
+            'sent_at' => now()->toIso8601String(),
+        ];
+    }
+}