Fix notebook markdown soft line breaks

[devmcp]

Markdown cells in notebooks were rendering soft line breaks as hard paragraph breaks when the Quarto Markdown-It renderer was active. The Pandoc div rule reported every line as a paragraph interrupter during Markdown-It silent validation, so hard-wrapped prose was split before normal Markdown rendering could preserve soft break semantics.

This updates the div parser so silent validation only reports an interruption for actual ::: div markers and does not mutate parser state until the marker is consumed. The new core tests cover both the soft-break regression and the expected paragraph interruption for Pandoc div markers.

Fixes #1017

Tests:

• npx --yes yarn@1.22.22 workspace core test
• npx --yes yarn@1.22.22 workspace quarto-vscode-markdownit build
• npx --yes yarn@1.22.22 build-vscode

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[juliasilge]

Thank you so much for this fix, @devmcp! I notice it is still draft, but it looks ready to go to me. Do you have any other changes you want to make before I review?

[devmcp]

Thank you so much for this fix, @devmcp! I notice it is still draft, but it looks ready to go to me. Do you have any other changes you want to make before I review?

Nope! I think it's good to review. Many thanks!

[juliasilge]

Thank you for this contribution! 🚀