Feature (canvas): Add mind tagging support (#11359)

### What problem does this PR solve?
Resolve the issue of missing thinking labels when viewing pre-existing
conversations
### Type of change

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

api/db/services/canvas_service.py

@@ -223,6 +223,10 @@ def completion(tenant_id, agent_id, session_id=None, **kwargs):
         ans["session_id"] = session_id
         if ans["event"] == "message":
             txt += ans["data"]["content"]
+            if ans["data"].get("start_to_think", False):
+                txt += "<think>"
+            elif ans["data"].get("end_to_think", False):
+                txt += "</think>"
         yield "data:" + json.dumps(ans, ensure_ascii=False) + "\n\n"
 
     conv.message.append({"role": "assistant", "content": txt, "created_at": time.time(), "id": message_id})