Unpin a2a library.

[polina-c]

Failure: https://github.com/flutter/genui/actions/runs/20315316405

Repro (for packages/genui):

1. Unpin a2a in pubspec.yaml (replace a2a: 3.2.0 with a2a: ^3.2.0)
2. Run flutter test --test-randomize-ordering-seed=39720092

Issue on a2a: shamblett/a2a#63