Fix: grow reference list (#9674)

### What problem does this PR solve?

Fix Multiple conversations cause the reference list to grow indefinitely
due to Python's mutable default argument behavior.
Explicitly initialize reference as empty list when creating new sessions

### Type of change

- [x] Bug Fix (non-breaking change which fixes an issue)

api/db/services/canvas_service.py

@@ -182,7 +182,8 @@ def completion(tenant_id, agent_id, session_id=None, **kwargs):
             "user_id": user_id,
             "message": [],
             "source": "agent",
-            "dsl": cvs.dsl
+            "dsl": cvs.dsl,
+            "reference": []
         }
         API4ConversationService.save(**conv)
         conv = API4Conversation(**conv)