fix: issue #5600 (#5645)

fix: issue https://github.com/infiniflow/ragflow/issues/5600

### What problem does this PR solve?

close issue https://github.com/infiniflow/ragflow/issues/5600 

### Type of change

- [x] Bug Fix (non-breaking change which fixes an issue)

graphrag/general/community_reports_extractor.py

                 response = re.sub(r"\{\{", "{", response)
                 response = re.sub(r"\}\}", "}", response)
                 logging.debug(response)
-                response = json.loads(response)
+                try:
+                    response = json.loads(response)
+                except json.JSONDecodeError as e:
+                    logging.error(f"Failed to parse JSON response: {e}")
+                    logging.error(f"Response content: {response}")
+                    continue
                 if not dict_has_keys_with_types(response, [
                             ("title", str),
                             ("summary", str),

graphrag/general/leiden.py

         result = {}
         results_by_level[level] = result
         for node_id, raw_community_id in node_id_to_community_map[level].items():
+            if node_id not in graph.nodes:
+                logging.warning(f"Node {node_id} not found in the graph.")
+                continue
             community_id = str(raw_community_id)
             if community_id not in result:
                 result[community_id] = {"weight": 0, "nodes": []}