Removal/Modification of response headers

**Is your feature request related to a problem? Please describe.**
Noticing that when trying to use response streaming via API Gateway, the underlying AgentCore mechanisms (Starlette?) is buffering and returning Content-Length header which appears to be possibly negating the API gateway response streaming capability (HTTP proxy) .

**Describe the solution you'd like**
Ability to modify content headers or remove Content-Length, etc when required.

未指定文件、测试或入口点。首先跟踪 Python SDK 的 API Gateway 集成中的响应流式传输和响应标头生成，然后确定如何在不禁用流式传输的情况下修改或删除标头。完成的标准是调用方可以控制相关标头，包括在需要时删除 Content-Length。

由索引模型根据 Issue 内容生成。