References in typst/tex are not added to the bib if they are only in the abstract

[rowanc1]

Now that we aren't processing the parts together, the references don't get exported correctly if they are only in a part.

[fwkoch]

I haven't recreated this to be sure, but I speculate the references are getting processed correctly, they just don't get written out to the bibtex file. If that's the case, all we need to do is pass the parts content in here, alongside the "selected file" content: https://github.com/jupyter-book/mystmd/blob/main/packages/myst-cli/src/build/typst.ts#L237