test ListUserPosts use case

5 cases: zero/negative userId -> BadRequest; user with no posts
-> []; returns only requested user's posts (filters out other
authors); ordered newest-first by createdAt. fails red.

backend/tests/Unit/Post/UseCases/ListUserPostsTest.php

@@ -0,0 +1,95 @@
+<?php
+
+namespace Tests\Unit\Post\UseCases;
+
+use App\Exceptions\BadRequestException;
+use App\Post\CreatePostDto;
+use App\Post\UseCases\ListUserPosts\ListUserPosts;
+use App\Post\UseCases\ListUserPosts\ListUserPostsRequest;
+use DateTimeImmutable;
+use DateTimeZone;
+use Tests\Fakes\FakePostRepository;
+use Tests\TestCase;
+
+class ListUserPostsTest extends TestCase
+{
+    private FakePostRepository $postRepo;
+
+    private DateTimeImmutable $now;
+
+    private ListUserPosts $useCase;
+
+    protected function setUp(): void
+    {
+        $this->now = new DateTimeImmutable(
+            '2026-05-06T12:00:00',
+            new DateTimeZone('UTC')
+        );
+        $this->postRepo = new FakePostRepository;
+        $this->useCase = new ListUserPosts($this->postRepo);
+    }
+
+    private function seedPost(int $userId, string $title, string $offset): void
+    {
+        $this->postRepo->create(new CreatePostDto(
+            userId: $userId,
+            title: $title,
+            body: 'body',
+            createdAt: $this->now->modify($offset),
+        ));
+    }
+
+    public function test_zero_user_id_throws_bad_request(): void
+    {
+        $this->expectException(BadRequestException::class);
+        $this->useCase->execute(new ListUserPostsRequest(userId: 0));
+    }
+
+    public function test_negative_user_id_throws_bad_request(): void
+    {
+        $this->expectException(BadRequestException::class);
+        $this->useCase->execute(new ListUserPostsRequest(userId: -1));
+    }
+
+    public function test_user_with_no_posts_returns_empty_array(): void
+    {
+        $this->seedPost(2, 'other-user-post', '-1 hour');
+
+        $posts = $this->useCase->execute(
+            new ListUserPostsRequest(userId: 1)
+        );
+
+        $this->assertSame([], $posts);
+    }
+
+    public function test_returns_only_requested_users_posts(): void
+    {
+        $this->seedPost(1, 'mine-1', '-2 days');
+        $this->seedPost(2, 'theirs', '-1 day');
+        $this->seedPost(1, 'mine-2', '-1 hour');
+
+        $posts = $this->useCase->execute(
+            new ListUserPostsRequest(userId: 1)
+        );
+
+        $this->assertCount(2, $posts);
+        foreach ($posts as $post) {
+            $this->assertSame(1, $post->getUserId());
+        }
+    }
+
+    public function test_returns_posts_ordered_newest_first(): void
+    {
+        $this->seedPost(1, 'first', '-3 days');
+        $this->seedPost(1, 'second', '-2 days');
+        $this->seedPost(1, 'third', '-1 hour');
+
+        $posts = $this->useCase->execute(
+            new ListUserPostsRequest(userId: 1)
+        );
+
+        $this->assertSame('third', $posts[0]->getTitle());
+        $this->assertSame('second', $posts[1]->getTitle());
+        $this->assertSame('first', $posts[2]->getTitle());
+    }
+}