Set up Reqnroll for BDD testing

src/Core/API/API.Specs/API.Specs.csproj

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+<Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <TargetFramework>net10.0</TargetFramework>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <Nullable>enable</Nullable>
+    <IsPackable>false</IsPackable>
+    <RootNamespace>API.Specs</RootNamespace>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.12.0" />
+    <PackageReference Include="xunit" Version="2.9.2" />
+    <PackageReference Include="xunit.runner.visualstudio" Version="2.8.2" />
+    <PackageReference Include="FluentAssertions" Version="6.9.0" />
+
+    <!-- Reqnroll core, xUnit adapter and code-behind generator -->
+    <PackageReference Include="Reqnroll" Version="3.3.3" />
+    <PackageReference Include="Reqnroll.xUnit" Version="3.3.3" />
+    <PackageReference Include="Reqnroll.Tools.MsBuild.Generation" Version="3.3.3" PrivateAssets="all" />
+
+    <!-- ASP.NET Core integration testing -->
+    <PackageReference Include="Microsoft.AspNetCore.Mvc.Testing" Version="9.0.1" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <!-- Ensure feature files are included in the project -->
+    <None Include="Features\**\*.feature" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <Using Include="Xunit" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <ProjectReference Include="..\API.Core\API.Core.csproj" />
+  </ItemGroup>
+</Project>

src/Core/API/API.Specs/Features/NotFound.feature

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+Feature: NotFound API
+  As a client of the API
+  I want consistent 404 responses
+  So that consumers can handle missing routes
+
+  Scenario: GET error 404 returns NotFound message
+    Given the API is running
+    When I GET "/error/404"
+    Then the response status code should be 404
+    And the response JSON should have "message" equal "Route not found."

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);
+    }
+}

src/Core/API/API.Specs/reqnroll.json

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+{
+  "$schema": "https://raw.githubusercontent.com/reqnroll/Reqnroll/main/Reqnroll.Configuration/reqnroll.schema.json",
+  "language": {
+    "feature": "en-US"
+  },
+  "bindingCulture": {
+    "name": "en-US"
+  },
+  "trace": {
+    "level": "Verbose"
+  },
+  "runtime": {
+    "stopAtFirstError": false
+  }
+}