Fix session.ask return generator bug when stream=False on python sdk

[Vela-zz]

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)