update method
Updates a running Live Activity with the full desired state.
ActivityKit replaces content-state wholesale, so payload must contain the
complete desired state, not a partial diff.
On iOS this fails for an id this app session didn't start: the handle needed to update an activity is process-local, so the update genuinely did not happen.
Implementation
Future<void> update(String activityId, LiveActivityPayload payload) {
throw UnimplementedError('update() has not been implemented.');
}