Update contact lookup sample

[nan-yu]

Tested:

• The contact_lookup client successfully connected to the contact_lookup agent and rendered the response correctly

Description

Replace this paragraph with a description of what this PR is changing or adding, and why. Consider including before/after screenshots.

List which issues are fixed by this PR. For larger changes, raising an issue first helps reduce redundant work.

Pre-launch Checklist

• I signed the CLA.
• I read the Contributors Guide.
• I read the Style Guide.
• I have added updates to the CHANGELOG.
• I updated/added relevant documentation.
• My code changes (if any) have tests.

If you need help, consider asking for advice on the discussion board.

[gemini-code-assist]

Code Review

This pull request introduces a significant and well-structured refactoring to create a centralized a2ui-agent package for managing A2UI schemas. The changes include a build-time hook to package schema files, a robust schema manager with fallback loading mechanisms, and a validator. The contact lookup sample has been updated to use this new package, which greatly improves maintainability and decouples the agent from hardcoded schemas. The overall approach is excellent. I've identified a few critical and high-severity issues related to exception handling and a NameError that could break the code at runtime, along with some medium-severity suggestions to improve the robustness of the build script and validation logic.

[zeroasterisk]

LGTM — clean CI improvements and schema packing hook. 👍

[nan-yu]

Rebased and ready for another look.