CmdPal: Ensure CommandItemViewModel reacts to changes of replaced Command (#42982)

## Summary of the Pull Request

This PR resolves the issue with CommandItemViewModel's subscription to
changes in the associated Command when it gets replaced by another
Command. The current implementation removes the handler from the old
command but fails to attach a new one.

## Change log one-liner


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## PR Checklist

- [x] Closes: #42981 
- [ ] **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
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pipeline](https://github.com/microsoft/PowerToys/blob/main/.pipelines/ci/templates/build-powertoys-steps.yml)
for new test projects
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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

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## Detailed Description of the Pull Request / Additional comments

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Author: jiripolasek

src/modules/cmdpal/Core/Microsoft.CmdPal.Core.ViewModels/CommandItemViewModel.cs

@@ -306,6 +306,7 @@ protected virtual void FetchProperty(string propertyName)
                 Command.PropertyChanged -= Command_PropertyChanged;
                 Command = new(model.Command, PageContext);
                 Command.InitializeProperties();
+                Command.PropertyChanged += Command_PropertyChanged;
 
                 // Extensions based on Command Palette SDK < 0.3 CommandItem class won't notify when Title changes because Command
                 // or Command.Name change. This is a workaround to ensure that the Title is always up-to-date for extensions with old SDK.

src/modules/cmdpal/ext/SamplePagesExtension/Pages/SampleListPage.cs

@@ -190,7 +190,11 @@ public override IListItem[] GetItems()
                     new CommandContextItem(new EverChangingCommand("Faces", "😁", "🥺", "😍")),
                     new CommandContextItem(new EverChangingCommand("Hearts", "♥️", "💚", "💜", "🧡", "💛", "💙")),
                 ],
-            }
+            },
+            new ListItemChangingCommandInTime()
+            {
+                Title = "I'm a list item that changes entire command in time",
+            },
         ];
     }
 
@@ -248,6 +252,11 @@ internal sealed partial class EverChangingCommand : InvokableCommand, IDisposabl
         private int _currentIndex;
 
         public EverChangingCommand(string name, params string[] icons)
+            : this(name, TimeSpan.FromSeconds(5), icons)
+        {
+        }
+
+        public EverChangingCommand(string name, TimeSpan interval, params string[] icons)
         {
             _icons = icons ?? throw new ArgumentNullException(nameof(icons));
             if (_icons.Length == 0)
@@ -260,7 +269,7 @@ public EverChangingCommand(string name, params string[] icons)
             Icon = new IconInfo(_icons[_currentIndex]);
 
             // Start timer to change icon and name every 5 seconds
-            _timer = new Timer(OnTimerElapsed, null, TimeSpan.FromSeconds(5), TimeSpan.FromSeconds(5));
+            _timer = new Timer(OnTimerElapsed, null, interval, interval);
         }
 
         private void OnTimerElapsed(object state)
@@ -282,4 +291,29 @@ public void Dispose()
             _timer?.Dispose();
         }
     }
+
+    internal sealed partial class ListItemChangingCommandInTime : ListItem
+    {
+        private readonly EverChangingCommand[] _commands =
+        [
+            new("Water", TimeSpan.FromSeconds(2), "🐬", "🐳", "🐟", "🦈"),
+            new("Faces", TimeSpan.FromSeconds(2), "😁", "🥺", "😍"),
+            new("Hearts", TimeSpan.FromSeconds(2), "♥️", "💚", "💜", "🧡", "💛", "💙"),
+        ];
+
+        private int _state;
+
+        public ListItemChangingCommandInTime()
+        {
+            Subtitle = "I change my command every 10 seconds, and the command changes it's icon every 2 seconds";
+            var timer = new Timer(OnTimerElapsed, null, TimeSpan.FromSeconds(10), TimeSpan.FromSeconds(10));
+            this.Command = _commands[0];
+        }
+
+        private void OnTimerElapsed(object state)
+        {
+            _state = (_state + 1) % _commands.Length;
+            this.Command = _commands[_state];
+        }
+    }
 }