refactor: remove explicit output definition and update chat return type hint

app.py

@@ -13,8 +13,8 @@ client = OpenAI(
     }
 )
 
-@app.api(outputs=["text"])
-def chat(message: str, history_json: list):
+@app.api()
+def chat(message: str, history_json: list) -> str:
     # history_json should be a list of dictionaries like [{"role": "user", "content": "..."}, {"role": "assistant", "content": "..."}]
     messages = history_json + [{"role": "user", "content": message}]