#776 App action as activity: ActivityContext missing current_step_name -> DataShard task log write fails

When an app action (e.g. apps.platform.simple_activity_test.compute_sum) runs as an activity (timeout>30s -> activity worker -> ActivityContext), enterprise/apps/executor.py _write_datashard_logs reads ctx.current_step_name (line 945) which only DurableContext defines -> AttributeError caught as 'Failed to write DataShard logs'. So the app's task_logs DataShard row is not written. Fix: add current_step_name (None default) to ActivityContext to satisfy the DurableContext interface; the code then falls back to app.action for the step name. Other executor ctx reads are already guarded (_managed_connection handles ActivityContext).