[Feat]: SSE Streaming providing to to_a2a

[sanjeevamk1]

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

• I agree to follow this project's Code of Conduct

[holtskinner]

Is this issue related to the A2A Integrations of Agent Development Kit (ADK)? If so, please re-file this issue in https://github.com/google/adk-python