fix beamer regression

[cscheid]

Closes #6936.

In order to be able to create RawBlock elements in LaTeX more robustly, I added a pass that coalesced divs in latex format ahead of merging consecutive RawBlocks. I missed the fact that some Pandoc writers in LaTeX subformats like Beamer check for classes in the div output. Now, we only coalesce divs if they're "purely structural".

[cscheid]

Note the lack of smoke tests.

There are a couple of issues here.

• our smoke-all suite doesn't support rendering to beamer while producing only .tex support, so _quarto: tests: ... doesn't work
• the broken filter happens very late in our processing, and so we can't actually check for the fix with a document Lua filter. (This is another argument for allowing Lua filters to be inserted anywhere in the chain.)
• Designing the correct test infra will take longer than fixing this obvious bug, so I'm merging a regressionless fix here while I create the test infra.