docs(fix[ext{sphinx_fonts}]): skip font_faces entry on download failure

why: font_faces.append() ran unconditionally, emitting @font-face CSS
rules pointing to missing files when download failed (browser 404s).
what:
- Move font_faces.append() inside the if _download_font() block
- Update test to expect 0 entries on download failure

Author: tony

docs/_ext/sphinx_fonts.py

@@ -95,14 +95,14 @@ def _on_builder_inited(app: Sphinx) -> None:
                 url = _cdn_url(package, version, font_id, subset, weight, style)
                 if _download_font(url, cached):
                     shutil.copy2(cached, fonts_dir / filename)
-                font_faces.append(
-                    {
-                        "family": font["family"],
-                        "style": style,
-                        "weight": str(weight),
-                        "filename": filename,
-                    }
-                )
+                    font_faces.append(
+                        {
+                            "family": font["family"],
+                            "style": style,
+                            "weight": str(weight),
+                            "filename": filename,
+                        }
+                    )
 
     preload_hrefs: list[str] = []
     preload_specs: list[tuple[str, int, str]] = app.config.sphinx_font_preload

tests/docs/_ext/test_sphinx_fonts.py

@@ -262,7 +262,7 @@ def test_on_builder_inited_download_failure(
     tmp_path: pathlib.Path,
     monkeypatch: pytest.MonkeyPatch,
 ) -> None:
-    """_on_builder_inited still builds font_faces entry on download failure."""
+    """_on_builder_inited skips font_faces entry on download failure."""
     monkeypatch.setattr("sphinx_fonts._cache_dir", lambda: tmp_path / "cache")
 
     msg = "offline"
@@ -285,8 +285,7 @@ def fake_urlretrieve(url: str, filename: t.Any) -> t.NoReturn:
 
     sphinx_fonts._on_builder_inited(app)
 
-    assert len(app._font_faces) == 1
-    assert app._font_faces[0]["family"] == "Inter"
+    assert len(app._font_faces) == 0
 
 
 def test_on_builder_inited_explicit_subset(