chore(spec): Update default protocol version to 0.3.0

[a2a-bot]

Release-As: 0.3.0

Protocol Update: a2aproject/A2A@6a152a8

[gemini-code-assist]

Summary of Changes

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This pull request updates the A2A specification to version 0.3.0, primarily focusing on enhancing security definitions and capabilities. Key changes include the introduction of skill-specific security requirements, support for mutual TLS authentication, and the ability to specify OAuth 2.0 metadata URLs. These updates improve the flexibility and robustness of how agents and their skills handle authentication and authorization.

Highlights

• Protocol Version Update: The A2A protocol version has been incremented from 0.2.6 to 0.3.0, indicating a significant update to the API specification.
• Granular Skill Security: Individual AgentSkill definitions can now specify their own security requirements, providing more granular control over access to specific agent capabilities.
• New Security Scheme: A new MutualTlsSecurityScheme has been added, expanding the range of supported authentication methods to include mutual TLS.
• OAuth2 Metadata Support: The OAuth2SecurityScheme now supports an oauth2_metadata_url, allowing for dynamic discovery of OAuth 2.0 authorization server metadata.

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[gemini-code-assist]

Code Review

This pull request updates the generated Python code for protobuf messages and Pydantic types to align with a new version of the A2A specification. The changes introduce new security schemes like mTLS, add metadata URLs for OAuth2, and update the protocol version. The modifications across a2a_pb2.py, a2a_pb2.pyi, and a2a/types.py are consistent with each other and appear to be correctly generated. I have reviewed the changes and found no issues.