fix: handle null response in LLM and improve JSON parsing in agent (#13187)

Fixes AttributeError in _remove_reasoning_content() when LLM returns
None, and improves JSON parsing regex for markdown code fences in
agent_with_tools.py

api/db/services/llm_service.py

@@ -265,6 +265,8 @@ class LLMBundle(LLM4Tenant):
             generation.end()
 
     def _remove_reasoning_content(self, txt: str) -> str:
+        if txt is None:
+            return None
         first_think_start = txt.find("<think>")
         if first_think_start == -1:
             return txt