fix: update problem_list population to use append for titles and document names

--bug=1064074 --user=刘瑞斌 【工作流知识库】高级分段时，文档名称、分段标题作为问题异常，被切割成单字问题了 https://www.tapd.cn/62980211/s/1806119

Author: liuruibin

apps/application/flow/step_node/document_split_node/impl/base_document_split_node.py

@@ -151,9 +151,9 @@ def _generate_problem_list(
                 problem_list.append(document_name)
         elif split_strategy == 'custom':
             if paragraph_title_relate_problem and paragraph.get('title'):
-                problem_list.extend(paragraph.get('title'))
+                problem_list.append(paragraph.get('title'))
             if document_name_relate_problem and document_name:
-                problem_list.extend(document_name)
+                problem_list.append(document_name)
         elif split_strategy == 'qa':
             if document_name_relate_problem and document_name:
                 problem_list.append(document_name)