Add regression test for and fix use of accent property for mo tag.

[rtibbles]

Summary

The pydantic implementation of the mo element I had previously done was missing the non-standard accent property. This was causing errors during publish for one of Peter's test channels.

I took the LaTeX from that specific exercise to make a regression test, and confirmed that adding the accent property to the pydantic class resolved the issue.

References

Fixes this error I saw when running publish in an interactive shell in a disposable pod on hotfixes:

ValidationError: 1 validation error for Mo
accent
  Extra inputs are not permitted [type=extra_forbidden, input_value='true', input_type=str]
    For further information visit https://errors.pydantic.dev/2.11/v/extra_forbidden

Reviewer guidance

Cross check with the MDN page: https://developer.mozilla.org/en-US/docs/Web/MathML/Reference/Element/mo#accent

I also asked Claude Code to do a follow up scan of all the MathML and HTML classes and compare them to the spec to see if anything else might be missing. It said nothing was, although I don't take that as proof positive! But at least, there's some validation that we have complete coverage now.

I think this would be fairly low risk to go into the search recommendations release - but it is probably simpler to reserve this for an immediate follow up patch release.

[marcellamaki]

I've cross referenced this with the mozilla docs (linked in PR description), and discussed my confusion about the non-standard browser support with Richard to ensure I was understanding the docs correctly. Seems okay to merge.