Move lib/crewai/src/crewai/a2a to lib/crewai-a2a/src/crewai_a2a as a
separate workspace member. crewai's `[a2a]` optional extra now pulls in
crewai-a2a instead of listing raw deps. All imports updated from
crewai.a2a.* to crewai_a2a.*. Handle PydanticUndefinedAnnotation in
crewai/__init__.py model_rebuild to break circular import at load time.
