[Testing] HttpRequest make body only bytes

Migration from https://github.com/aws/aws-durable-execution-sdk-python-testing/issues/64

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## overview
Follow-up from this PR https://github.com/aws/aws-durable-execution-sdk-python-testing/pull/59/files

Refactor HttpRequest so body only is type bytes, and not dict.

## acceptance criteria
1. Make HttpRequest body type bytes
2. Amend from_bytes to just update the bytes as you have it here in from_raw_bytes
3. Add a body_to_dict instance method that contains all the deserialization parsing. might need to add a _operation_name to the instance. This is what the handlers will call then, instead of "parse_json_body"

Empieza por HttpRequest, comparando from_bytes con from_raw_bytes, y luego rastrea los handlers que actualmente llaman a parse_json_body. Actualiza el manejo del cuerpo de la solicitud para que siga siendo bytes y asegúrate de que body_to_dict contenga la deserialización y de que los handlers lo utilicen.

Escrito por el modelo de indexación a partir del texto del issue.