fix: update A2A extension HTTP header

[wgd3]

Description

This PR updates the HTTP header name for extension support. It is currently defined as X-A2A-Extensions, however as of this recent commit that has changed:

• a2aproject/A2A@c56ea72
• docs: Update A2A-Extensions header to align with current standard A2A#1248
• Extension Header Conflicting Names A2A#1238

Changes

The primary change comes from the src/a2a/extensions/common.py file:

HTTP_EXTENSION_HEADER = 'A2A-Extensions'
HTTP_EXTENSION_HEADER_DEPRECATED = 'X-A2A-Extensions'

Minor functional changes were made in the JSON-RPC app:

if not extension_values:
    extension_values = request.headers.getlist(
        HTTP_EXTENSION_HEADER_DEPRECATED
    )

Additional Notes

There is also support for the deprecated X-A2A-Extensions header built-in as part of this PR. I'm happy to remove or enhance this support if necessary.

---

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[gemini-code-assist]

Summary of Changes

Hello @wgd3, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request addresses an upstream change in the A2A project by updating the HTTP header used for extension support. The core purpose is to transition to the new 'A2A-Extensions' header while maintaining robust backward compatibility with the previously used 'X-A2A-Extensions' header. This ensures that services can adopt the new standard without immediately breaking existing client integrations, providing a smooth transition path.

Highlights

• HTTP Header Update: The primary HTTP header for A2A extensions has been updated from 'X-A2A-Extensions' to 'A2A-Extensions' to align with recent changes in the upstream A2A project.
• Backward Compatibility: Support for the deprecated 'X-A2A-Extensions' header has been explicitly added to ensure backward compatibility for existing clients.
• Service Integration: Both JSON-RPC and gRPC server request handlers have been updated to prioritize the new header while gracefully falling back to the deprecated header if the new one is not present.
• Test Updates: Relevant test cases and their docstrings have been updated to reflect the change to the new 'A2A-Extensions' header.

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[gemini-code-assist]

Code Review

This pull request correctly updates the HTTP header for A2A extensions from X-A2A-Extensions to the standard A2A-Extensions. The implementation thoughtfully includes backward compatibility to support the deprecated header, ensuring a smooth transition. The changes are consistent across the JSON-RPC and gRPC handlers, and the tests have been updated accordingly. I've provided a couple of minor suggestions to improve code conciseness, but overall, this is a solid and well-executed change.