fix: Resolve 4 pylint CI errors

- Add LinAlgError import in groupby_regression_optimized.py
- Fix test imports to use make_parallel_fit_v4 (v1 doesn't exist)
- Rename duplicate function in test_groupby_regression.py

Author: miranov25

UTILS/dfextensions/groupby_regression/groupby_regression.py

@@ -80,7 +80,7 @@ def make_linear_fit(
                         group_dict[f"{target_col}_slope_{col}"] = model.coef_[i]
                     group_dict[f"{target_col}_intercept"] = model.intercept_
                 except Exception as e:
-                    logging.warning(f"Linear regression failed for {target_col} in group {group_vals}: {e}")
+                    logging.warning(f"Linear regression failed for {target_col} in group {groupby_key}: {e}")
                     for col in linear_columns:
                         group_dict[f"{target_col}_slope_{col}"] = np.nan
                     group_dict[f"{target_col}_intercept"] = np.nan