test GetPost use case

4 cases: zero/negative id -> BadRequest; unknown id -> null
(controller maps to 404); existing id returns the Post.
GetPost takes int id directly (no Request object - the value is
trivial and controllers pull it from a route param).

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

@@ -0,0 +1,66 @@
+<?php
+
+namespace Tests\Unit\Post\UseCases;
+
+use App\Exceptions\BadRequestException;
+use App\Post\CreatePostDto;
+use App\Post\Post;
+use App\Post\UseCases\GetPost\GetPost;
+use DateTimeImmutable;
+use DateTimeZone;
+use Tests\Fakes\FakePostRepository;
+use Tests\TestCase;
+
+class GetPostTest extends TestCase
+{
+    private FakePostRepository $postRepo;
+
+    private DateTimeImmutable $now;
+
+    private GetPost $useCase;
+
+    protected function setUp(): void
+    {
+        $this->now = new DateTimeImmutable(
+            '2026-05-06T12:00:00',
+            new DateTimeZone('UTC')
+        );
+        $this->postRepo = new FakePostRepository;
+        $this->useCase = new GetPost($this->postRepo);
+    }
+
+    public function test_zero_id_throws_bad_request(): void
+    {
+        $this->expectException(BadRequestException::class);
+        $this->useCase->execute(0);
+    }
+
+    public function test_negative_id_throws_bad_request(): void
+    {
+        $this->expectException(BadRequestException::class);
+        $this->useCase->execute(-5);
+    }
+
+    public function test_unknown_id_returns_null(): void
+    {
+        $result = $this->useCase->execute(999);
+
+        $this->assertNull($result);
+    }
+
+    public function test_existing_id_returns_post(): void
+    {
+        $created = $this->postRepo->create(new CreatePostDto(
+            userId: 7,
+            title: 'My Post',
+            body: 'Some body content.',
+            createdAt: $this->now,
+        ));
+
+        $result = $this->useCase->execute($created->getId());
+
+        $this->assertInstanceOf(Post::class, $result);
+        $this->assertSame('My Post', $result->getTitle());
+        $this->assertSame(7, $result->getUserId());
+    }
+}