[Feat]: Update demo app with new a2a-python sdk

### Is your feature request related to a problem? Please describe.

Is there any plan to update demo app with the new a2a-python sdk ? seems lot changed.

https://github.com/google/A2A/tree/main/demo

### Describe the solution you'd like

updated demo app which works with new a2a python sdk

### Describe alternatives you've considered

_No response_

### Additional context

_No response_

### Code of Conduct

- [x] I agree to follow this project's Code of Conduct