Test.cs

public class ExchangeRateClientTests
{
    private readonly MemoryCache _cache;

    public ExchangeRateClientTests()
    {
        _cache = new MemoryCache(new MemoryCacheOptions());
    }
    
    [Fact]
    public async Task Rate()
    {
        var exchangeRate = 1.5m;
        
        var httpClient = new HttpClient(new TestHttpHandler(Currency.CAD, exchangeRate));
        var obj = new ExchangeRateClient(httpClient, _cache);
        
        var result = await obj.Rate(new DateOnly(2022, 12, 01), Currency.USD, Currency.CAD);
        
        Assert.Equal(exchangeRate, result);
    }
}

public class TestHttpHandler : HttpMessageHandler
{
    private readonly Currency _toCurrency;
    private readonly decimal _exchangeRate;

    public TestHttpHandler(Currency toCurrency, decimal exchangeRate)
    {
        _toCurrency = toCurrency;
        _exchangeRate = exchangeRate;
    }
    
    protected override Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
    {
        var response = new HttpResponseMessage(HttpStatusCode.OK);
        var json = JsonConvert.SerializeObject(new { Rates = new Dictionary<string, decimal> { { _toCurrency.ToString(), _exchangeRate } } });
        response.Content = new StringContent(json);
        return Task.FromResult(response);
    }
}