Fix:Fail to get list_sessions (#7678)

### What problem does this PR solve?

Close #7655

Based on the codes atthe api_app, I think the reference is one-to-one
with the message
`
    def fillin_conv(ans):
        nonlocal conv, message_id
        if not conv.reference:
            conv.reference.append(ans["reference"])
        else:
            conv.reference[-1] = ans["reference"]
conv.message[-1] = {"role": "assistant", "content": ans["answer"], "id":
message_id}
        ans["id"] = message_id
`



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

api/apps/sdk/session.py

         if conv["reference"]:
             messages = conv["messages"]
             message_num = 0
-            chunk_num = 0
             while message_num < len(messages):
                 if message_num != 0 and messages[message_num]["role"] != "user":
                     chunk_list = []
-                    if "chunks" in conv["reference"][chunk_num]:
-                        chunks = conv["reference"][chunk_num]["chunks"]
+                    if "chunks" in conv["reference"][message_num]:
+                        chunks = conv["reference"][message_num]["chunks"]
                         for chunk in chunks:
                             new_chunk = {
                                 "id": chunk.get("chunk_id", chunk.get("id")),
                             }
 
                             chunk_list.append(new_chunk)
-                    chunk_num += 1
                     messages[message_num]["reference"] = chunk_list
                 message_num += 1
         del conv["reference"]