Test.cs

public static class VinDecoderFixture
{
    public static VinDecoder CreateSuccessful(
        int year = 2026,
        string make = "Ford",
        string model = "F-150")
    {
        var json = $$"""
                     {
                       "year": {{year}},
                       "make": "{{make}}",
                       "model": "{{model}}",
                     }
                     """;

        var handler = new StubHttpMessageHandler(_ =>
            new HttpResponseMessage(HttpStatusCode.OK)
            {
                Content = new StringContent(json, Encoding.UTF8, "application/json")
            });

        var httpClient = new HttpClient(handler);
        return new VinDecoder(httpClient);
    }

    public static VinDecoder CreateFailure(
        HttpStatusCode statusCode = HttpStatusCode.BadRequest,
        string json = """{ "error": "invalid request" }""")
    {
        var handler = new StubHttpMessageHandler(_ =>
            new HttpResponseMessage(statusCode)
            {
                Content = new StringContent(json, Encoding.UTF8, "application/json")
            });

        var httpClient = new HttpClient(handler);
        return new VinDecoder(httpClient);
    }
}

public class Tests
{
    [Fact]
    public async Task Handle_returns_vehicle_name()
    {
        var vinDecoder = VinDecoderFixture.CreateSuccessful(
            year: 2026,
            make: "Ford",
            model: "F-150");

        var sut = new RegisterVehicleHandler(vinDecoder);

        var result = await sut.Handle(new RegisterVehicleCommand("123456789"), CancellationToken.None);

        Assert.Equal("2026 Ford F-150", result);
    }
}