test: Get coverage for PandasLikeSeries.native

Wasn't planning to implement it yet but https://github.com/narwhals-dev/narwhals/actions/runs/13616393368/job/38059997094?pr=2130

Author: dangotbanned

narwhals/_pandas_like/series.py

@@ -177,20 +177,20 @@ def _from_iterable(
         )
 
     def __len__(self: Self) -> int:
-        return len(self._native_series)
+        return len(self.native)
 
     @property
     def name(self: Self) -> str:
         return self._name
 
     @property
     def dtype(self: Self) -> DType:
-        native_dtype = self._native_series.dtype
+        native_dtype = self.native.dtype
         return (
             native_to_narwhals_dtype(native_dtype, self._version, self._implementation)
             if native_dtype != "object"
             else object_native_to_narwhals_dtype(
-                self._native_series, self._version, self._implementation
+                self.native, self._version, self._implementation
             )
         )