[Task]: Research removing buf from build process #706

Currently buf is executed on each build.

Given that the spec isn't going to be updated often after the release 99% of runs will be no-op while it requires more dependencies to install for initial uv sync/uv run pytest. We should consider extracting it into a separate step which is going to be run manually when needed (when new spec update is coming).

In addition see #754 (comment) - currently one needs to run uv build in a cloned repo to generate 0.3 files as they are ignored. Considerations above apply to these files as well.

BUT: seems like this process is also required currently for a2a.json here. So maybe this file should be committed to the repo same way as the generated code to avoid regenerating each time (now it's in .gitignore). It's unclear if it works/matches main branch.

Given the above but proposed plan is:

1. Remove buf from build process and commit all required files.
2. Deal with but.