Fix mypy section-name validation type

The Python package workflow failed in the Ubuntu Python 3.14 job while running mypy. The error reported that GitConfigParser.add_section() may receive configparser's _SectionName type, which is `str | _UNNAMED_SECTION`, but _assure_config_name_safe() accepted only `str`.

Keep the runtime validation unchanged for normal string section and option names, but type the guard for configparser section names and only apply the unsafe-character regex to strings. This preserves compatibility with configparser's section-name type while still rejecting CR, LF, and NUL in real names.

Validation:
- uv run --with gitdb --with mypy --with pytest --with pytest-cov --with ddt --with pytest-mock --with pytest-instafail --with pytest-sugar --with pre-commit mypy --python-version=3.14
- uv run --with gitdb --with pytest --with pytest-cov --with ddt --with pytest-mock --with pytest-instafail --with pytest-sugar python -m pytest test/test_config.py::TestBase::test_set_value_rejects_unsafe_section_and_option_names --no-cov
- uv run --with ruff ruff check git/config.py

Author: codex

git/config.py

@@ -892,8 +892,8 @@ def _value_to_string_safe(self, value: Union[str, bytes, int, float, bool]) -> s
             raise ValueError("Git config values must not contain CR, LF, or NUL")
         return value_str
 
-    def _assure_config_name_safe(self, name: str, label: str) -> None:
-        if UNSAFE_CONFIG_NAME_RE.search(name):
+    def _assure_config_name_safe(self, name: "cp._SectionName", label: str) -> None:
+        if isinstance(name, str) and UNSAFE_CONFIG_NAME_RE.search(name):
             raise ValueError("Git config %s names must not contain CR, LF, or NUL" % label)
 
     @needs_values