[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"

从 HttpRequest 开始，对比 from_bytes 和 from_raw_bytes，然后跟踪当前调用 parse_json_body 的处理程序。更新请求体处理，使其保持为 bytes，并确保 body_to_dict 包含反序列化逻辑，且处理程序使用它。

由索引模型根据 Issue 内容生成。