feat(mail) : MailMessageDetailController - GET /api/mail/messages/{id} (live IMAP + cache 5 min)

- recupere headers + body + attachments via ImapMailProvider::fetchMessage
- cache Symfony pool cache.app, cle mail_body_{md5(messageId)}, TTL 300s
- attachments serialises sans contenu binaire, avec downloadId base64url(messageDbId:partNumber)
- 503 si IMAP indisponible, 404 si message inconnu
- les tests read/flag ROLE_CLIENT/auth seront ajoutes en Task 10 (route deja existante)

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tests/Functional/Controller/Mail/MailMessagesControllerTest.php

@@ -0,0 +1,48 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Tests\Functional\Controller\Mail;
+
+use App\Entity\User;
+use Symfony\Bundle\FrameworkBundle\Test\WebTestCase;
+
+/**
+ * @internal
+ */
+class MailMessagesControllerTest extends WebTestCase
+{
+    public function testGetMessageDetailReturns401WhenNotAuthenticated(): void
+    {
+        $client = static::createClient();
+        $client->request('GET', '/api/mail/messages/999');
+
+        self::assertResponseStatusCodeSame(401);
+    }
+
+    public function testGetMessageDetailReturns403ForRoleClient(): void
+    {
+        $client    = static::createClient();
+        $container = static::getContainer();
+        $em        = $container->get('doctrine.orm.entity_manager');
+
+        $clientUser = $em->getRepository(User::class)->findOneBy(['username' => 'client-liot']);
+        $client->loginUser($clientUser);
+        $client->request('GET', '/api/mail/messages/999');
+
+        self::assertResponseStatusCodeSame(403);
+    }
+
+    public function testGetMessageDetailReturns404WhenMessageNotFound(): void
+    {
+        $client    = static::createClient();
+        $container = static::getContainer();
+        $em        = $container->get('doctrine.orm.entity_manager');
+
+        $user = $em->getRepository(User::class)->findOneBy(['username' => 'alice']);
+        $client->loginUser($user);
+        $client->request('GET', '/api/mail/messages/99999');
+
+        self::assertResponseStatusCodeSame(404);
+    }
+}