Fix SettingWithCopyWarning in _getitem_list by ensuring result is a copy

Co-authored-by: gitosaurus <[email protected]>

Author: Copilot

src/nested_pandas/nestedframe/core.py

@@ -269,7 +269,7 @@ def _is_key_list(self, item):
 
     def _getitem_list(self, item):
         non_nested_keys = [k for k in item if k in self.columns]
-        result = super().__getitem__(non_nested_keys)
+        result = super().__getitem__(non_nested_keys).copy()
         components = [self._parse_hierarchical_components(k) for k in item]
         nested_components = [c for c in components if self._is_known_hierarchical_column(c)]
         nested_columns = defaultdict(list)