save chat history to jupyter lab root dir

The /export command was saving  chat history in the pwd, i.e., the directory from where JupyterLab is launched rather than the JupyterLab root. For JupyterLab sessions, where the root directory is different from the launch directory, users will not be able to see the exported chat history file in JupyterLab. This is now rectified.

packages/jupyter-ai/jupyter_ai/chat_handlers/export.py

@@ -26,7 +26,8 @@ def chat_message_to_markdown(self, message):
             return ""
 
     # Multiple chat histories in separate files
-    def get_chat_filename(self, path="./chat_history.md"):
+    def get_chat_filename(self, file_name="chat_history.md"):
+        path = os.path.join(self.root_dir, file_name)
         filename, extension = os.path.splitext(path)
         counter = 1
         while os.path.exists(path):