Fix: Initialize first_stage_tasks and entity_relation_task to prevent empty-task cancel errors

- Initialize first_stage_tasks = [] and entity_relation_task = None at coroutine start
- Ensure cancel block safely handles no-op when tasks lists are empty

lightrag/lightrag.py

@@ -1229,6 +1229,8 @@ class LightRAG:
                     async with semaphore:
                         nonlocal processed_count
                         current_file_number = 0
+                        first_stage_tasks = []
+                        entity_relation_task = None
                         try:
                             # Get file path from status document
                             file_path = getattr(