fix(config): Restore html_favicon default dropped in workspace refactor

why: Commit cc813db explicitly added html_favicon="_static/favicon.ico"
as a shared default; it was accidentally dropped in the workspace
split (82a1441). The test asserting its absence was written after the
accidental removal, documenting the bug rather than the intent.
what:
- Restore "html_favicon": "_static/favicon.ico" to merge_sphinx_config()
- Fix test_merge_sphinx_config_html_paths to assert the correct value

Author: tony

packages/gp-sphinx/src/gp_sphinx/config.py

@@ -375,6 +375,7 @@ def merge_sphinx_config(
         # Static files and templates
         "html_static_path": list(DEFAULT_HTML_STATIC_PATH),
         "templates_path": list(DEFAULT_TEMPLATES_PATH),
+        "html_favicon": "_static/favicon.ico",
         # Theme (gp-sphinx child theme provides sidebars, templates, CSS, JS)
         "html_theme": DEFAULT_THEME,
         "html_theme_path": [],

tests/test_config.py

@@ -381,7 +381,7 @@ def test_merge_sphinx_config_html_paths() -> None:
     )
     assert result["html_static_path"] == ["_static"]
     assert result["templates_path"] == ["_templates"]
-    assert "html_favicon" not in result
+    assert result["html_favicon"] == "_static/favicon.ico"
     assert "html_extra_path" not in result