#775 Converge the two ActivityWorker implementations (enterprise vs engine package)

Live worker is enterprise/services/activity_worker.py (hwe activity-worker). A parallel refactored impl exists at engine/services/activity_worker/ (execution.py+worker.py, used by hwec). They diverged: the engine one already had the H-7 tenant_id emit fix, the live enterprise one did not (fixed in #772). Converge to one to avoid future drift.