Set up Reqnroll for BDD testing

src/Core/API/API.Specs/Steps/ApiSteps.cs

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+using System.Net;
+using System.Net.Http.Json;
+using Reqnroll;
+using FluentAssertions;
+
+namespace API.Specs.Steps;
+
+[Binding]
+public class ApiSteps
+{
+    private readonly TestApiFactory _factory;
+    private HttpClient? _client;
+    private HttpResponseMessage? _response;
+
+    public ApiSteps()
+    {
+        _factory = new TestApiFactory();
+    }
+
+    [Given("the API is running")]
+    public void GivenTheApiIsRunning()
+    {
+        _client = _factory.CreateClient();
+    }
+
+    // No user service assumptions needed for 404 tests
+
+    [When("I GET {string}")]
+    public async Task WhenIGet(string path)
+    {
+        _client.Should().NotBeNull("API client must be initialized");
+        _response = await _client!.GetAsync(path);
+    }
+
+    [Then("the response status code should be {int}")]
+    public void ThenStatusCodeShouldBe(int expected)
+    {
+        _response.Should().NotBeNull();
+        ((int)_response!.StatusCode).Should().Be(expected);
+    }
+
+    [Then("the response JSON should have {string} equal {string}")]
+    public async Task ThenResponseJsonShouldHaveFieldEqual(string field, string expected)
+    {
+        _response.Should().NotBeNull();
+        var dict = await _response!.Content.ReadFromJsonAsync<Dictionary<string, object>>();
+        dict.Should().NotBeNull();
+        dict!.TryGetValue(field, out var value).Should().BeTrue();
+        (value?.ToString()).Should().Be(expected);
+    }
+}