build: pin A2A spec to avoid breaking ci

[guglielmo-san]

Description

This PR a2aproject/A2A#1507 introduces a breaking change in the specification which is going to be addressed separately.

[gemini-code-assist]

Summary of Changes

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This pull request addresses an issue where a recent breaking change in the a2aproject/A2A specification was causing CI failures. To mitigate this, the buf.gen.yaml configuration has been updated to pin the external specification dependency to a known stable commit, ensuring the build process remains functional while the breaking change is resolved separately.

Highlights

• Dependency Pinning: Pinned the a2aproject/A2A Git repository reference in buf.gen.yaml to a specific commit hash (aca981cee3e7a3f22a4df8fb8a5302406f7a1cf5) to prevent a breaking change introduced in a recent update to the A2A specification.

Changelog

• buf.gen.yaml
  • Updated the ref for the a2aproject/A2A Git repository to a specific commit hash.

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[gemini-code-assist]

Code Review

The pull request effectively addresses the immediate issue of a breaking change in the A2A specification by pinning the buf.gen.yaml reference to a specific commit hash. This ensures the stability of the CI pipeline, which is a crucial step to prevent build failures.