fix(model_selection): pin KFold seed in knn cross-validation tests

Under --all-features the std_rand feature makes an unseeded KFold draw
OS entropy, so every CI run shuffled the folds differently. A sweep of
20k seeds showed 0.17% of shuffles violate the MAE < 10.0 assertion in
test_cross_val_predict_knn (worst 12.81) and 0.01% violate
train_score < test_score in test_cross_validate_knn.

Pin seed Some(11) in both tests; margins are comfortable (predict MAE
2.84 vs 10, validate test 2.71 vs 15). The entropy path stays covered
by the rand_custom tests.

Author: Mec-iS

src/model_selection/mod.rs

@@ -451,6 +451,9 @@ mod tests {
 
         let cv = KFold {
             n_splits: 5,
+            // Pinned seed: under std_rand an unseeded KFold draws OS entropy,
+            // which makes these assertions flaky across CI runs.
+            seed: Some(11),
             ..KFold::default()
         };
 
@@ -500,6 +503,9 @@ mod tests {
 
         let cv: KFold = KFold {
             n_splits: 2,
+            // Pinned seed: under std_rand an unseeded KFold draws OS entropy,
+            // which makes these assertions flaky across CI runs.
+            seed: Some(11),
             ..KFold::default()
         };