CmdPal: Fix regression when updating a command provider without commands (#40984)

Improves item insertion logic in TopLevelCommandManager.

Updated the insertion logic to handle invalid startIndex values. If
startIndex is -1, new items will be appended to the end of the
collection, enhancing robustness.

Fixes regression introduced in #40752

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src/modules/cmdpal/Microsoft.CmdPal.UI.ViewModels/TopLevelCommandManager.cs

@@ -170,20 +170,29 @@ public partial class TopLevelCommandManager : ObservableObject,
             // TODO: just added a lock around all of this anyway, but keeping the clone
             // while looking on some other ways to improve this; can be removed later.
             List<TopLevelViewModel> clone = [.. TopLevelCommands];
-            var startIndex = -1;
 
+            var startIndex = FindIndexForFirstProviderItem(clone, sender.ProviderId);
+            clone.RemoveAll(item => item.CommandProviderId == sender.ProviderId);
+            clone.InsertRange(startIndex, newItems);
+
+            ListHelpers.InPlaceUpdateList(TopLevelCommands, clone);
+        }
+
+        return;
+
+        static int FindIndexForFirstProviderItem(List<TopLevelViewModel> topLevelItems, string providerId)
+        {
             // Tricky: all Commands from a single provider get added to the
             // top-level list all together, in a row. So if we find just the first
             // one, we can slice it out and insert the new ones there.
-            for (var i = 0; i < clone.Count; i++)
+            for (var i = 0; i < topLevelItems.Count; i++)
             {
-                var wrapper = clone[i];
+                var wrapper = topLevelItems[i];
                 try
                 {
-                    if (sender.ProviderId == wrapper.CommandProviderId)
+                    if (providerId == wrapper.CommandProviderId)
                     {
-                        startIndex = i;
+                        return i;
-                        break;
                     }
                 }
                 catch
@@ -191,9 +200,8 @@ public partial class TopLevelCommandManager : ObservableObject,
                 }
             }
 
-            clone.RemoveAll(item => item.CommandProviderId == sender.ProviderId);
+            // If we didn't find any, then we just append the new commands to the end of the list.
-            clone.InsertRange(startIndex, newItems);
+            return topLevelItems.Count;
-            ListHelpers.InPlaceUpdateList(TopLevelCommands, clone);
         }
     }