test(docs[sphinx_fonts]): add tests for sphinx_fonts extension

why: codecov drops because docs/_ext/sphinx_fonts.py is measured
for coverage but has zero tests across all repos.
what:
- Add test_sphinx_fonts.py with 21 tests covering all functions
- Test pure functions, I/O with monkeypatch, Sphinx events with SimpleNamespace
- Cover all branches: cached/success/URLError/OSError, html/non-html, empty/with fonts

Author: tony