Fix: Handle reverse-sorted lists with duplicates in presorted check

- Updated `_handlePresorted` and '_handlePresortedBy' to treat non-increasing sequences (e.g., [3, 2, 2, 1])
  as reverse-sorted, allowing O(n) reversal instead of O(n log n) sorting.

Author: abbashosseinii

pkgs/collection/lib/src/algorithms.dart

@@ -587,7 +587,7 @@ bool _handlePresorted<E>(
   if (compare(elements[start], elements[start + 1]) > 0) {
     // Check strictly decreasing
     var i = start + 1;
-    while (i < end && compare(elements[i - 1], elements[i]) > 0) {
+    while (i < end && compare(elements[i - 1], elements[i]) >= 0) {
       i++;
     }
     if (i == end) {
@@ -771,7 +771,7 @@ bool _handlePresortedBy<E, K>(List<E> elements, K Function(E) keyOf,
     int Function(K, K) compare, int start, int end) {
   if (compare(keyOf(elements[start]), keyOf(elements[start + 1])) > 0) {
     var i = start + 1;
-    while (i < end && compare(keyOf(elements[i - 1]), keyOf(elements[i])) > 0) {
+    while (i < end && compare(keyOf(elements[i - 1]), keyOf(elements[i])) >= 0) {
       i++;
     }
     if (i == end) {