fix: Add return type hint -> str to chat endpoint to prevent UserWarning and enable client data reception

app.py

@@ -18,7 +18,7 @@ async def get_hf_token_status():
     return {"has_token": token_exists}
 
 @app.api(name="chat")
-async def chat(messages_json: str, temperature: float = 0.7, max_tokens: int = 1024, custom_token: str = None):
+async def chat(messages_json: str, temperature: float = 0.7, max_tokens: int = 1024, custom_token: str = None) -> str:
     # Check for Hugging Face token: custom override or environment variable
     hf_token = (custom_token and custom_token.strip()) or os.environ.get("HF_TOKEN")