chore: Update agent.py to fix prompt handling and remove unused import

src/server/agent.py

@@ -3,7 +3,7 @@ from typing import Any, List, Sequence, Union
 from langchain_core.prompts import PromptTemplate
 from langchain.agents import AgentExecutor, create_tool_calling_agent,create_structured_chat_agent
 from langchain.tools import BaseTool
-from langgraph.prebuilt import create_react_agent
+# from langgraph.prebuilt import create_react_agent
 
 
 from langchain_core.language_models import BaseLanguageModel
@@ -115,9 +115,8 @@ class Agent:
         self.prompt = prompt
 
         if not prompt:
-            agent = create_react_agent(llm, tools,debug=verbose)
-            self.agent_executor = agent
-        else:
+            raise ValueError("PromptTemplate is required")
+        
         agent = create_chart_agent(llm, tools, prompt, **args)
         self.agent_executor = AgentExecutor(agent=agent,tools=tools,verbose=verbose)