Ensure a2ui data part can represent multiple changes applied atomically

[jacobsimionato]

A "Part" is the atomic unit of a message that can be streamed by an agent across A2A.

In A2UI, we want to make sure that an agent can atomically apply any possible update to the UI, ideally avoiding introducing a "Commit" message etc, because that adds complexity to the spec.

E.g. perhaps we want to wrap the entire server_to_client message schema in an array?