Implement Dart A2UI agent SDK library.

Use blueprint: https://github.com/a2ui-project/a2ui/blob/main/blueprints/modules/a2ui_agent.blueprint.md

Library name: a2ui_agent

- [ ] 0.9
- [x] json_schema_builder: https://github.com/flutter/genui/pull/1029
- [ ] core: https://github.com/a2ui-project/a2ui/issues/2373
- [x] main PR: https://github.com/a2ui-project/a2ui/pull/2439
- [ ] follow up for https://github.com/a2ui-project/a2ui/pull/2439#discussion_r3928080732: https://github.com/a2ui-project/a2ui/pull/2538
- [ ] agent:
- [ ] Prototype for API and tests: https://github.com/a2ui-project/a2ui/pull/2408
- [ ] Prepare tests: https://github.com/a2ui-project/a2ui/pull/2633
- [ ] 1.0 (blocked by not-readiness of 1.0 blueprint)
- [ ] core
- [ ] agent

The issue references a blueprint at https://github.com/a2ui-project/a2ui/blob/main/blueprints/modules/a2ui_agent.blueprint.md and multiple pull requests (2439, 2408, 2633). Start by reading the blueprint to understand the library's design. Then review the linked PRs to see the current state of the core and agent modules. The work involves implementing an SDK library, which requires understanding the existing codebase and the A2UI architecture.

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