#370 MEDIUM: _save_state not called automatically before sleep/commit

## Issue The _save_state() method in engine/durable_context.py:1462-1496 persists executed_tasks, failed_tasks, sleeping_tasks but is NOT automatically called before sleep/commit. State may be lost if not explicitly called. ## Location engine/durable_context.py:1462-1496 ## Fix Integrate _save_state() into transaction commit path to ensure state is always persisted