Add tests to reach 100% code coverage (#412)

Summary:
Pull Request resolved: #412

Add ~70 new test methods across 10 test files covering all previously uncovered lines (110 lines) in the balance package. Coverage was at 98% with gaps in balance_frame.py, sample_frame.py, sample_class.py, balancedf_class.py, cli.py, summary_utils.py, model_matrix.py, rake.py, weighted_comparisons_plots.py, and weighted_comparisons_stats.py.

Key areas covered:
- FutureWarning deprecation paths for id_column/weight_column property changes
- Error validation branches in set_fitted_model, design_matrix, predict_proba, predict_weights
- IPW model helper methods (_require_ipw_model, _matrix_to_dataframe, _align_to_index, _ipw_class_index)
- Training weights fallback logic in _resolve_design_weights
- SampleFrame.from_frame edge cases (string column args, missing covariates, covar/id overlap warning)
- SampleFrame.rename_weight_column and from_sample validation
- BalanceDF base class methods and BalanceDFOutcomes/Weights error paths
- build_design_matrix scaler, penalty rescaling, and matrix type conversion branches
- Rake validation for proportions (zero, NaN, inf, negative, non-float)
- ASMD unknown std_type and r_indicator scalar reshaping
- Empty variables early return in seaborn_plot_dist
- Fix StandardScaler copy=False read-only array bug in model_matrix.py

Differential Revision: D100949347

fbshipit-source-id: a02fe16b8c3d4acbb288f5e35881cedc5882b615

Author: talgalili

balance/utils/model_matrix.py

@@ -823,7 +823,7 @@ def build_design_matrix(
     if fit_scaler is not None:
         combined_matrix = fit_scaler.transform(combined_matrix)
     elif scaler_weights is not None:
-        scaler = StandardScaler(with_mean=False, copy=False)
+        scaler = StandardScaler(with_mean=False)
         combined_matrix = scaler.fit_transform(
             combined_matrix, sample_weight=scaler_weights
         )