Set system.text.json.jsonserializer globally minimal api

ConfigureHttpJsonOptions and AddJsonOptions manage serilizer settings for model binding/response serialization, they will not affect the settings you use to deserialize manually. Provide settings for JsonSerializer.Deserialize, for example:

var res = JsonSerializer.Deserialize<WeatherForecast[]>(responseContent, 
    new JsonSerializerOptions
    {
        PropertyNameCaseInsensitive = true
    });

Notes

1. There is no need to explicitly configure JSON options for API to only set the PropertyNameCaseInsensitive to true since JsonSerializerDefaults.Web defaults to PropertyNameCaseInsensitive = true:

   The following options have different defaults for web apps:

   • PropertyNameCaseInsensitive = true
   • JsonNamingPolicy = CamelCase
   • NumberHandling = AllowReadingFromString

2. You can resolve configured JSON options from the DI. For example for minimal APIs (or you can configure your own and resolve them):

   app.MapGet("/weatherforecast", (IOptions<Microsoft.AspNetCore.Http.Json.JsonOptions> opts) =>
        {
            var serOpts = opts.Value.SerializerOptions;
            // ...
        });
   
   

3. You should not need to deserialize data to just serialize it back again (in the shown code at least). Returning content should do the trick: return Results.Content(responseContent, "application/json");

4. You can't mutate default serialization options (see this github comment for example), at least ATM, but you can define helper/extension methods which will use some global statically defined ones.