add non-stream mode support to session.ask function

### What problem does this PR solve?

same as title, I do not know why the stream=False is not work on the
server side also, when stream=False, the response in the
session._ask_chat is a fully connnected SSE string.

This is a quick fix on the sdk side to make the response format align
with API docs

### Type of change

- [x] Bug Fix (non-breaking change which fixes an issue)