test: add failing authenticate user test

Red phase: write AuthenticateUserTest with cases for valid credentials, empty email/password (null and empty string), unknown email, wrong password, and fresh instance guarantee. Fakes included.

backend/tests/Fakes/FakeUserRepository.php

@@ -0,0 +1,63 @@
+<?php
+
+namespace Tests\Fakes;
+
+use App\Shared\ValueObject\EmailAddress;
+use App\User\CreateUserDto;
+use App\User\User;
+use App\User\UserRepository;
+
+class FakeUserRepository implements UserRepository
+{
+    private array $users = [];
+
+    public function create(CreateUserDto $dto): User
+    {
+        $id = $this->nextId();
+
+        $user = new User(
+            id: $id,
+            email: $dto->email,
+            passwordHash: $dto->passwordHash,
+        );
+
+        $this->users[$id] = $user;
+
+        return $user;
+    }
+
+    public function findByEmail(EmailAddress $email): ?User
+    {
+        foreach ($this->users as $user) {
+            if ($user->getEmail()->value() === $email->value()) {
+                return new User(
+                    id: $user->getId(),
+                    email: $user->getEmail(),
+                    passwordHash: $user->getPasswordHash(),
+                );
+            }
+        }
+
+        return null;
+    }
+
+    public function find(int $id): ?User
+    {
+        $user = $this->users[$id] ?? null;
+
+        if ($user === null) {
+            return null;
+        }
+
+        return new User(
+            id: $user->getId(),
+            email: $user->getEmail(),
+            passwordHash: $user->getPasswordHash(),
+        );
+    }
+
+    private function nextId(): int
+    {
+        return count($this->users);
+    }
+}