fix: Fix Simple Application preview show 404 error page

--bug=1052069 --user=刘瑞斌 【简易应用】- 调试预览中提问报404 https://www.tapd.cn/57709429/s/1651941

apps/application/flow/tools.py

@@ -18,6 +18,10 @@ from common.response import result
 
 class Reasoning:
     def __init__(self, reasoning_content_start, reasoning_content_end):
+        if not reasoning_content_start:
+            reasoning_content_start = "<think>"
+        if not reasoning_content_end:
+            reasoning_content_end = "</think>"
         self.content = ""
         self.reasoning_content = ""
         self.all_content = ""