ENH: expose font_file in brain.add_text (#12543)

[PragnyaKhandelwal]

Reference issue (if any)

Fixes #12543.

What does this implement/fix?

This PR exposes a font_file parameter in brain.add_text() and forwards it to the underlying PyVista add_text() call.

This allows users to pass a .ttf/.ttc font file for Unicode glyph rendering (for example, CJK characters and symbols), while preserving current behavior when font_file is not provided.

Additional information

Verification context:

1. Aligns with issue discussion where specifying font_file in PyVista resolves Unicode rendering for add_text().
2. Keeps scope minimal by exposing existing backend capability in the MNE API.

[wmvanvliet]

Really nice work! It would be nice to also test setting font_file as part of one of the unit tests.

[PragnyaKhandelwal]

Thanks for the review @wmvanvliet . I updated the backend to always pass font_file to Plotter.add_text, including None and added a unit test that checks both cases: font_file set to a non-None value and font_file=None.

[PragnyaKhandelwal]

I verified the checks failed....looks like the build_docs failure is due to the ci merge step issue not a doc/content failure from this pr

[PragnyaKhandelwal]

Hi @wmvanvliet and @drammock, just a quick note that I've addressed the feedback regarding the font discovery logic and documentation clarifications.
I've verified the font passthrough locally. Whenever you have a moment, I’m standing by for a final review or any further adjustments needed to get this merged!

[wmvanvliet]

One comment left unaddressed. Why are you using a kwargs dict instead of just passing the parameters to the function? Also, could you please disclose AI usage for this PR?

[PragnyaKhandelwal]

One comment left unaddressed. Why are you using a kwargs dict instead of just passing the parameters to the function? Also, could you please disclose AI usage for this PR?

Well good catch, @wmvanvliet. I’ve switched from kwargs passthrough to explicit parameter passing to match MNE style.
Also, yes, I used Gemini as drafting assistance for parts of this PR, then manually reviewed/edited the final changes. I should have disclosed this earlier and will do so clearly in future PRs.

[PragnyaKhandelwal]

At this point, only ci/circleci: build_docs is failing (looks like a CI timeout during docs build), while the other checks are passing. Could you please re-run that job when convenient?