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.

No file, test, or entry point is named. Start by tracing response streaming and response-header generation in the Python SDK's API Gateway integration, then determine how headers can be modified or removed without disabling streaming. Done means callers can control relevant headers, including removing Content-Length when needed.

Written by the indexing model from the issue text.