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

<!-- Enter a brief description/summary of your PR here. What does it
fix/what does it change/how was it tested (even manually, if necessary)?
-->
## Summary of the Pull Request

<!-- Please review the items on the PR checklist before submitting-->
## PR Checklist

- [ ] Closes: #xxx
- [ ] **Communication:** I've discussed this with core contributors
already. If the work hasn't been agreed, this work might be rejected
- [ ] **Tests:** Added/updated and all pass
- [ ] **Localization:** All end-user-facing strings can be localized
- [ ] **Dev docs:** Added/updated
- [ ] **New binaries:** Added on the required places
- [ ] [JSON for
signing](https://github.com/microsoft/PowerToys/blob/main/.pipelines/ESRPSigning_core.json)
for new binaries
- [ ] [WXS for
installer](https://github.com/microsoft/PowerToys/blob/main/installer/PowerToysSetup/Product.wxs)
for new binaries and localization folder
- [ ] [YML for CI
pipeline](https://github.com/microsoft/PowerToys/blob/main/.pipelines/ci/templates/build-powertoys-steps.yml)
for new test projects
- [ ] [YML for signed
pipeline](https://github.com/microsoft/PowerToys/blob/main/.pipelines/release.yml)
- [ ] **Documentation updated:** If checked, please file a pull request
on [our docs
repo](https://github.com/MicrosoftDocs/windows-uwp/tree/docs/hub/powertoys)
and link it here: #xxx

<!-- Provide a more detailed description of the PR, other things fixed,
or any additional comments/features here -->
## Detailed Description of the Pull Request / Additional comments

<!-- Describe how you validated the behavior. Add automated tests
wherever possible, but list manual validation steps taken as well -->
## Validation Steps Performed

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;
-                        break;
+                        return i;
                     }
                 }
                 catch
@@ -191,9 +200,8 @@ public partial class TopLevelCommandManager : ObservableObject,
                 }
             }
 
-            clone.RemoveAll(item => item.CommandProviderId == sender.ProviderId);
-            clone.InsertRange(startIndex, newItems);
-            ListHelpers.InPlaceUpdateList(TopLevelCommands, clone);
+            // If we didn't find any, then we just append the new commands to the end of the list.
+            return topLevelItems.Count;
         }
     }