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.

tests/Feature/MailTest/MailTestControllerTest.php

@@ -0,0 +1,59 @@
+<?php
+
+namespace Tests\Feature\MailTest;
+
+use App\Domains\MailTest\Mailables\TestMail;
+use Illuminate\Support\Facades\Mail;
+use Tests\TestCase;
+
+class MailTestControllerTest extends TestCase
+{
+    public function test_it_sends_a_test_email(): void
+    {
+        Mail::fake();
+
+        $response = $this->postJson('/api/mail-test/send', [
+            'to' => 'recipient@example.com',
+            'subject' => 'SMTP test',
+            'message' => 'Test message',
+        ]);
+
+        $response->assertOk()
+            ->assertJsonPath('message', 'Correo de prueba enviado correctamente.')
+            ->assertJsonPath('recipient', 'recipient@example.com')
+            ->assertJsonPath('mailer', 'array')
+            ->assertJsonStructure(['sent_at']);
+
+        Mail::assertSent(TestMail::class, function (TestMail $mail): bool {
+            return $mail->hasTo('recipient@example.com')
+                && $mail->mailSubject === 'SMTP test'
+                && $mail->mailMessage === 'Test message';
+        });
+    }
+
+    public function test_it_uses_default_content_when_optional_fields_are_omitted(): void
+    {
+        Mail::fake();
+
+        $this->postJson('/api/mail-test/send', [
+            'to' => 'recipient@example.com',
+        ])->assertOk();
+
+        Mail::assertSent(TestMail::class, function (TestMail $mail): bool {
+            return $mail->mailSubject === 'Prueba de correo de Shopit'
+                && $mail->mailMessage === 'Este es un correo de prueba enviado desde Shopit.';
+        });
+    }
+
+    public function test_it_validates_the_recipient(): void
+    {
+        Mail::fake();
+
+        $this->postJson('/api/mail-test/send', [
+            'to' => 'invalid-email',
+        ])->assertUnprocessable()
+            ->assertJsonValidationErrors(['to']);
+
+        Mail::assertNothingSent();
+    }
+}