Fix: truncate message issue. (#5765)

### What problem does this PR solve?

Close #5761

### Type of change

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

rag/prompts.py

     if c < max_length:
         return c, msg
 
-    msg_ = [m for m in msg[:-1] if m["role"] == "system"]
+    msg_ = [m for m in msg if m["role"] == "system"]
     if len(msg) > 1:
         msg_.append(msg[-1])
     msg = msg_