Update tests.py

tests.py

@@ -237,7 +237,7 @@ def run_code(language:str,packages:str,filename: str, code: str,start_cmd:str,fo
         - bash/apk packages cannot be installed.
         - When editing and subsequently re-executing the server with the forever_cmd='true' setting, the previous server instance will be automatically terminated, and the updated server will commence operation. This functionality negates the requirement for manual process termination commands such as pkill node.    
         - The opened ports can be externally accessed at https://suitable-liked-ibex.ngrok-free.app/ (ONLY if the website is running successfully)
-
+        - Do not use `plt.show()` in this headless environment. Save visualizations directly (e.g., `plt.savefig("happiness_img.png")` or export GIFs/videos).  
     """
     global destination_dir
     package_names = packages.strip()