Jupyter Runtime dir disappears

Hi,

I noticed that the Jupyter Runtime dir (`/run/user/$UID/jupyter`) disappears sometimes.

So I have to recreate it to prevent a Kernel error (`Failed to start the kernel`) :

```python
FileNotFoundError: [Errno 2] No such file or directory: '/run/user/1000/jupyter/kernel-01da634f-a030-4bde-9e0a-d56944ebe60c.json'
```

Can you help ?

Start by reproducing the missing `/run/user/$UID/jupyter` directory and the `Failed to start the kernel` error, then trace the notebook runtime and kernel-startup entry points that create or access the directory. Done means the runtime directory and kernel connection remain available without manual recreation; no source files or tests are named in the issue.

Written by the indexing model from the issue text.