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/Requests/SendTestMailRequest.php

@@ -0,0 +1,25 @@
+<?php
+
+namespace App\Domains\MailTest\Requests;
+
+use Illuminate\Foundation\Http\FormRequest;
+
+class SendTestMailRequest extends FormRequest
+{
+    public function authorize(): bool
+    {
+        return true;
+    }
+
+    /**
+     * @return array<string, array<int, string>>
+     */
+    public function rules(): array
+    {
+        return [
+            'to' => ['required', 'string', 'email', 'max:255'],
+            'subject' => ['nullable', 'string', 'max:255'],
+            'message' => ['nullable', 'string', 'max:5000'],
+        ];
+    }
+}