feat: migrate timestamp extension to a2a 1.0

[ishymko]

1. Migrate to a2a 1.0 alpha 2.
2. Replace referencing this package from other samples with a folder-local test. Note: required fixing ruff for those files as they were added to diff and ruff got executed, seems like they were not clean before and had many ruff violations.

[gemini-code-assist]

Code Review

This pull request migrates the timestamp extension to version 1.0.0a2 of the a2a-sdk. Key changes include refactoring the extension to use raw Protobuf types instead of Pydantic models, updating the client interceptor to utilize the new before/after hooks, and adding a comprehensive end-to-end showcase test. The extension was also removed from several sample agents and hosts. Feedback focuses on two high-severity issues where the Python Protobuf API is misused: specifically, the attempt to use .append() on a repeated field and the incorrect initialization of nested message fields in constructors.