A2ui extension selection

[nan-yu]

Description

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Pre-launch Checklist

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• I updated/added relevant documentation.
• My code changes (if any) have tests.

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[gemini-code-assist]

Code Review

This pull request introduces a version selection mechanism for A2UI extensions, which is a valuable improvement for ensuring forward compatibility. The refactoring across multiple agent examples to support this new mechanism is extensive. However, I've identified a few critical bugs in the sample agent implementations that would lead to runtime errors, as well as a high-severity issue in the core version selection logic that could cause incorrect behavior or crashes. Additionally, there's an opportunity to improve the consistency of the refactoring pattern applied across the different agents for better maintainability.