优化DifyExporter类，新增按日期范围过滤消息的功能，支持UTC+8时区的时间转换，并更新导出文件名以包含日期范围信息。同时，重构消息处理逻辑

rag2_0/dify/dify_tool.py

@@ -40,13 +40,18 @@ class PgSql:
         如果连接失败，会抛出异常。
         """
         try:
+            self.DIFY_PG_USER = os.getenv("DIFY_PG_USER")
+            self.DIFY_PG_PASSWORD = os.getenv("DIFY_PG_PASSWORD")
+            self.DIFY_PG_HOST = os.getenv("DIFY_PG_HOST")
+            self.DIFY_PG_PORT = os.getenv("DIFY_PG_PORT")
+            self.DIFY_PG_DATABASE = os.getenv("DIFY_PG_DATABASE")
             # 连接数据库
             self.connection = psycopg2.connect(
-                user=os.getenv("DIFY_PG_USER"),
-                password=os.getenv("DIFY_PG_PASSWORD"),
-                host=os.getenv("DIFY_PG_HOST"),
-                port=os.getenv("DIFY_PG_PORT"),
-                database=os.getenv("DIFY_PG_DATABASE")
+                user=self.DIFY_PG_USER,
+                password=self.DIFY_PG_PASSWORD,
+                host=self.DIFY_PG_HOST,
+                port=self.DIFY_PG_PORT,
+                database=self.DIFY_PG_DATABASE
             )
 
         except (Exception, psycopg2.Error) as error:
@@ -273,7 +278,10 @@ class DifyTool:
                 return None
             workflow_node_executions_info = self.dify_pgsql.get_workflow_node_executions_info(messages_info['workflow_run_id'])
             if not workflow_node_executions_info:
-                return None
+                return {
+                "messages_info": messages_info,
+                "workflow_node_executions_info": None
+            }
             return {
                 "messages_info": messages_info,
                 "workflow_node_executions_info": workflow_node_executions_info