#713 ReflexiveOperator completes workflow before activity finishes

ReflexiveOperator calls execute_activity_operator which is queue-only, but doesn't wait for the activity completion event. This causes workflows to complete while the activity is still running. Root cause: - execute_activity_operator was refactored to be queue-only (to fix double-queue bug) - _execute_reflexive_operator creates an internal ActivityOperator - It doesn't add a WaitForEventOperator or explicit wait for completion Evidence from workflow run 73d23874-b01e-4568-9c52-f94088325f10: - Workflow completed at 20:58:08.315 - Activity started at 20:58:09.208 - Activity completed at 20:58:23.172 Fix: Add await_event() call after queuing to wait for activity completion, similar to _execute_as_activity pattern.