aws lambda + http api with Payload Format Version 2.0. How to correctly parse the payload with Python 3.12 runtime

I am developing a lambda (runtime Python 3.12) that will process events via POST requests to a API Gateway HTTP API.

According to the AWS console, the integration I have set up between the POST route on my API and my lambda function has "Payload format version 2.0 (interpreted response format).

Here is the documentation about that payload format.

That documentation gives me the (apparently wrong) impression that in the Python runtime, the "event" parameter passed to the lambda is a dict that has (among other things) an attribute 'body' that has a value that is a string.

Thus, based on that (again, apparently wrong) impression, my lambda function tries to access the data in the body of the request (which I expect to be a JSON formatted string) like this:

req_body = json.loads(event.body)

When I run a test of my function using the console, my function raises this exception:

{
  "errorMessage": "'dict' object has no attribute 'body'",
  "errorType": "AttributeError",
  "requestId": "83b129eb-cf1e-4afa-9215-a0f3c0d18cec",
  "stackTrace": [
    "  File \"/var/task/lambda_function.py\", line 33, in handler\n    req_body = json.loads(event.body)\n"
  ]
}

Here is a screenshot of the test that is leads to the raised exception:

What do I not correctly understand about the event object delivered to a Python 3.12 lambda function by an HTTP API using Payload format version 2.0?

Thanks.