[FEATURE]: Expose awaitable SurfaceController message-processing completion

- [x] I have searched the existing issues to make sure this feature has not already been requested.

## Is your feature request related to a problem? Please describe.

After submitting a parsed message, callers have no public signal that all message-owned asynchronous work—especially schema validation and resulting surface notifications—has completed. Flutter frame quiescence is not equivalent to controller transaction completion, creating races for replay, tests, and coherent observation.

## Describe the Proposed Solution

Expose an awaitable API such as Future handleMessageAsync(A2uiMessage message), or an equivalent completion token tied to the submitted message. Completion should mean the controller applied the message, awaited validation owned by that transaction, reconciled surface state, and published public notifications/diagnostics. The synchronous API can remain for compatibility.

## Describe Alternatives Considered

Pumping frames can be held open by unrelated animations and is not a validation boundary. Fixed delays are slow and flaky. Reading private controller state is unsupported.

## Additional Context

Related architectural work: #1877. This request is deliberately smaller and can land independently.