fix: breaking changes introduced in nbformat 3

[aslowwriter]

Explanation

Nbformat 3.0 and jupyter-protocol added some new enum variants, which is technically a braking change but really not a big deal.

General Checklist

• Updated tests or added new tests (if applicable)
• Branch is up to date with main
• Tests & lints hooks pass
• Updated documentation (if applicable)
• History of branch is cleaned up

[codecov]

Codecov Report

❌ Patch coverage is 14.28571% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.4%. Comparing base (822c263) to head (1dc6638).

Files with missing lines	Patch %	Lines
src/parsing/python/jupyter.rs	20.0%	4 Missing ⚠️
src/render/jupyter.rs	0.0%	2 Missing ⚠️

Additional details and impacted files

Files with missing lines	Coverage Δ
src/cli/mod.rs	89.4% <ø> (ø)
src/parsing/sphinx/types.rs	94.7% <ø> (ø)
src/render/jupyter.rs	66.6% <0.0%> (ø)
src/parsing/python/jupyter.rs	65.5% <20.0%> (-8.6%)	⬇️

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