Feat: support tree structured deep-research policy. (#12559)

### What problem does this PR solve?

#12558
### Type of change

- [x] New Feature (non-breaking change which adds functionality)

api/db/services/conversation_service.py

@@ -64,6 +64,7 @@ class ConversationService(CommonService):
             offset += limit
         return res
 
+
 def structure_answer(conv, ans, message_id, session_id):
     reference = ans["reference"]
     if not isinstance(reference, dict):
@@ -107,6 +108,7 @@ def structure_answer(conv, ans, message_id, session_id):
             conv.reference[-1] = reference
     return ans
 
+
 async def async_completion(tenant_id, chat_id, question, name="New session", session_id=None, stream=True, **kwargs):
     assert name, "`name` can not be empty."
     dia = DialogService.query(id=chat_id, tenant_id=tenant_id, status=StatusEnum.VALID.value)