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
- Add test infrastructure (conftest, __init__) for docs/_ext tests
- 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

docs/_ext/conftest.py

@@ -0,0 +1,10 @@
+"""Pytest configuration for docs/_ext doctests."""
+
+from __future__ import annotations
+
+import pathlib
+import sys
+
+_ext_dir = pathlib.Path(__file__).parent
+if str(_ext_dir) not in sys.path:
+    sys.path.insert(0, str(_ext_dir))

tests/docs/__init__.py

@@ -0,0 +1,3 @@
+"""Tests for documentation extensions."""
+
+from __future__ import annotations

tests/docs/_ext/__init__.py

@@ -0,0 +1,3 @@
+"""Tests for docs/_ext Sphinx extensions."""
+
+from __future__ import annotations

tests/docs/_ext/conftest.py

@@ -0,0 +1,10 @@
+"""Fixtures and configuration for docs extension tests."""
+
+from __future__ import annotations
+
+import pathlib
+import sys
+
+docs_ext_path = pathlib.Path(__file__).parent.parent.parent.parent / "docs" / "_ext"
+if str(docs_ext_path) not in sys.path:
+    sys.path.insert(0, str(docs_ext_path))