SSE Streaming providing to to_a2a

Is your feature request related to a problem? Please describe.
The current Streaming default is none, to have streaming decided at the start by the user in the to_a2a function, which will be an advantage while exposing the agent via a2a

Describe the solution you'd like
While creating the runner, the run config takes streaming_mode as none, so we need to update it so that it takes what the user gives for the streaming part

Describe alternatives you've considered
No response

Additional context
No response

Code of Conduct