How do I fix my error when deserializing json from HttpRequest in Azure Function App?

I've recently started using azure function apps. I am passing some json data from Postman into my app when it's running so that I can test it and I'm getting an error of an unexpected character '{'.

{
    "testId": "001",
    "shopItems": {
       "itemName": "something"
    }
}

Here is what is happening.

Firstly, I have my very basic function, which can be called via it's route in postman. I pass the above request in the body as JSON to this function which then calls a service to work with it.

[FunctionName("UpdateCart")]
public async Task<IActionResult> UpdateCart(
    [HttpTrigger(AuthorizationLevel.Function, "post", Route = "v1/cart/update")] HttpRequest req,
    ILogger log)
{
    await _myService.UpdateCartAsync(req.Body);

    //Return ok
    return new OkResult();
}

The body property of 'HttpRequest' is a 'Stream' and therefore my service and it's interface expect a stream to be passed in.

Interface

namespace MyProject.Services.Interface
{
    public interface IMyService
    {

        Task UpdateCartAsync(Stream contents);
    }
}

Service

Since we know it's a stream, I use "ReadToEndAsync" which then produces a string from the result. I pass that string to the "JsonConvert.Deserializer" to map that to my model and that's where it fails with the unexpected character '{' which is the bracer before "itemName".

public async Task UpdateCartAsync(Stream content)
{
        
    var result = await new StreamReader(content).ReadToEndAsync();

    Basket cart = JsonConvert.DeserializeObject<Cart>(result); //fails here

    var db = _connectionMultiplexer.GetDatabase();

    await db.StringSetAsync(cart.CartId, cart.CartContents);
}

Model

public class Cart
{
    public string CartId { get; set; }
    public string CartContents { get; set; }
}

I'm guessing here that the Steam string isn't actually accurate or that I perhaps need to serialize first, I'm not sure. Is there another way to pass JSON from the body to a functions app?