test: exempt stub_path from win32 cov gate

_CachedTool.stub_path is the one function here missing
`# pragma: win32 no cover`. pytestmark skips every test on win32, so
its body never runs there, and covdefaults' 100% gate caught it:
windows-2025 jobs failed at 99.53% while every other platform held
100%.

Verified against covdefaults' own pragma regex (keys off os.name /
sys.platform / sys.implementation.name): inert everywhere but win32,
confirmed by an unchanged 100%, 45-passed run on Linux.

Assisted-by: Sisyphus:claude-sonnet-5 opencode

Author: MaxymVlasov

tests/pytest/tool_version_test.py

@@ -72,7 +72,11 @@ class _CachedTool(NamedTuple):
     tool_dir: str
     bin_name: str
 
-    def stub_path(self, cache_root: Path, version: str) -> Path:
+    def stub_path(  # pragma: win32 no cover
+        self,
+        cache_root: Path,
+        version: str,
+    ) -> Path:
         """Build the `<root>/<tool>/<version>/<binary>` cache path.
 
         Args: