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