Fix: normalize kernel path with forward slashes (Windows)

[TimMonko]

References and Relevant Issues

Closes #3010

Description

When executing notebooks on Windows, the kernel starts from the root rather than the notebook's directory. On Unix systems, the kernel starts from the notebook's directory. This is because path.relative returns native separators (backslashes on Windows), and Jupyter Server API paths are /-delimited. With a Windows path, Jupyter Server silently falls back to the server root.

Jupyter server docs explaining the path resolution
And I think this is the actual juypter-server forward-slash code being executed by myst-execute.

I made https://github.com/TimMonko/myst-cwd-repro to demonstrate the issue -- and building with bun (my first time) and running this repo works with the fix.

I'm not sure if I can demonstrate this with a regression test because CI is only Linux it appears, so I guess you could mock the Windows path? Anyways, it's why I left a thorough comment in the code.

I used Deepseek to help me navigate mystmd. To be honest, I've never looked at typescript in my life so please be honest with me.

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[choldgraf]

This feels like a safe change to me, thanks for the comments and keeping this tightly focused

[agoose77]

Excellent — good spot that the paths are unix-like, not platform-specific!