[dart sdk] a2ui_core should depend on genai_primitives

We want a2ui_core to play well with ecosystem, so it should use genai_primitives.

Find the dependency configuration for a2ui_core, likely in a pubspec.yaml or similar Dart/Flutter package file. Understand how genai_primitives is defined and published. Update the dependency declaration, then verify the build and any imports in the a2ui_core codebase still work.

索引モデルが issue の本文から書いたものです。