Pin the TeX Live version in the PDF docs CI job

[chrisjsewell]

The build-doc-formats latex job uses xu-cheng/latex-action@v2, which pulls the unpinned texlive-full:latest image. That image drifting to TeX Live 2026 (after 2026-06-29) broke the PDF build on all branches with no change on our side (see #1154 for the diagnosis and workaround; upstream: sphinx-doc/sphinx#14465).

To stop the image drifting under us again:

• adopt ⬆️ Bump xu-cheng/latex-action from 2 to 3 #803 (dependabot bump of xu-cheng/latex-action v2 → v3; still open but stale — needs ·@·d·ependabot r·ebase or a manual recreate)
• set the v3 texlive_version input in .github/workflows/test-formats.yml to pin a known-good TeX Live release
• add a note/reminder to bump the pin deliberately (e.g. once a year), since a hard pin also stops us noticing genuine TeX Live incompatibilities early

Once pinned, workarounds like #1154's latex_table_style can be lifted on our own schedule (tracked in #1155) rather than being forced by an image update.