Update A2A sample notebooks (a2a_http_sample.ipynb and a2a_sdk_sample.ipynb) for General Availability (v1), remove pre-GA disclaimers, update workbench references, and correct Python setup syntax.

Update A2A sample notebooks (a2a_http_sample.ipynb and a2a_sdk_sample.ipynb) for General Availability (v1), remove pre-GA disclaimers, update workbench references, and correct Python setup syntax. #12890