public class UnitTest
{
    private readonly HttpClient _client = new(new MyHttpMessageHandler());

    [Fact]
    public async Task Test1()
    {
        Response? response = null;
        
        var options = MyJsonSerializerContext.Default.Options;
        
        var exception = await Record.ExceptionAsync(async () =>
        {
            response = await _client.GetFromJsonAsync<Response>("http://abc", options);
        });
        
        Assert.Null(exception);
        Assert.Equal(1234, response?.Code);
    }
    
    [Fact]
    public async Task Test2()
    {
        Response? response = null;
        
        var options = new JsonSerializerOptions()
        {
            TypeInfoResolver = MyJsonSerializerContext.Default,
        };
        
        var exception = await Record.ExceptionAsync(async () =>
        {
            response = await _client.GetFromJsonAsync<Response>("http://abc", options);
        });
        
        Assert.IsType<JsonException>(exception);
        Assert.StartsWith("The JSON value could not be converted to System.Int32. Path: $.Code", exception.Message);
    }
}

public class MyHttpMessageHandler : HttpMessageHandler
{
    protected override Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
    {
        return Task.FromResult(new HttpResponseMessage()
        {
            Content = new StringContent("{\"Code\": \"1234\"}"),
        });
    }
}
    
public class Response
{
    public int Code { get; set; }
}

[JsonSourceGenerationOptions(NumberHandling = JsonNumberHandling.AllowReadingFromString)]
[JsonSerializable(typeof(Response))]
public partial class MyJsonSerializerContext : JsonSerializerContext;