[v3.0.0bx] Use ContentsManager instead of local FS

[MaicoTimmerman]

Description

Jupyter-server uses jupyter_server.services.contents.manager.ContentsManager for interactions with the filesystem. This gives users the ability to attach external file systems to Jupyter, for example hosted on object storage (i.e. S3 or HDFS).

Now when using Jupyter-AI, the /generate and @file methods, will try to read from local filesystem. However, when an non-local Contents-Manager is used, the files never show up in the JupyterLab UI.

Reproduce

Not really relevant

Expected behavior

• For the UI to show contents generated by Jupyter-AI commands, such as /export and /generate.
• For the @file context provider to load files shown in the UI.