fix planner prompt template parsing

src/query/planner/service.py

@@ -11,6 +11,7 @@ from __future__ import annotations
 
 from pathlib import Path
 
+from langchain_core.messages import SystemMessage
 from langchain_core.prompts import ChatPromptTemplate
 from langchain_core.runnables import Runnable
 from langchain_openai import AzureChatOpenAI
@@ -47,7 +48,7 @@ def _build_default_chain() -> Runnable:
     )
     prompt = ChatPromptTemplate.from_messages(
         [
-            ("system", _load_prompt_text()),
+            SystemMessage(content=_load_prompt_text()),
             ("human", "{human_content}"),
         ]
     )