Refactor: Improve list generation in _generatePathological

Refine the list creation to be more explicit and performant.

- Use a more descriptive name `sortedBase` instead of `base`.
- Create the initial `sortedBase` list with `growable: false` as it is never modified.
- Use `List.of` and explicitly set `growable: true` for the final copies to make the intent clear that they are mutable.

Author: abbashosseinii

pkgs/collection/benchmark/dataset_generator.dart

@@ -40,14 +40,14 @@ List<List<int>> _generateFewUnique() {
 }
 
 List<List<int>> _generatePathological() {
-  final base = List.generate(size, (i) => i);
+  final sortedBase = List.generate(size, (i) => i, growable: false);
   // Creates a "V-shape" or "organ pipe" array that can be challenging
   // for quicksort implementations by promoting unbalanced partitions.
   final pathological = <int>[
-    for (int i = 0; i < size; i++)
-      if (i.isEven) base[i],
-    for (int i = size - 1; i > 0; i--)
-      if (i.isOdd) base[i],
+    for (var i = 0; i < size; i++)
+      if (i.isEven) sortedBase[i],
+    for (var i = size - 1; i > 0; i--)
+      if (i.isOdd) sortedBase[i],
   ];
-  return List.generate(count, (_) => List<int>.from(pathological));
+  return List.generate(count, (_) => List.of(pathological, growable: true));
 }