Fix for Unused local variable

Co-authored-by: Copilot Autofix powered by AI <62310815+github-advanced-security[bot]@users.noreply.github.com>

Author: hamidkm9

src/linearboost/linear_boost.py

@@ -863,6 +863,7 @@ def fit(self, X, y, sample_weight=None) -> Self:
                 # Store validation data (original features for kernel computation)
                 validation_data = (X_val_transformed, y_val, sample_weight_val)
                 y = y_train
+                assert y is not None
             else:
                 # For linear or approximate kernels, split after transformation
                 n_samples = training_data.shape[0]
@@ -891,6 +892,7 @@ def fit(self, X, y, sample_weight=None) -> Self:
                 # Store validation data for checking
                 validation_data = (training_data_val, y_val, sample_weight_val)
                 y = y_train
+                assert y is not None
         else:
             y_train = y