Fix: Prevent memory leak in view model lifecycle (#40216)

<!-- 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

A memory leak was identified where activating a Command Palette
extension via a hotkey would lead to a linear increase in resource
consumption and a corresponding decrease in search performance.

Each activation created a new `PageViewModel` instance for the
extension's UI. However, the `ShellViewModel` did not dispose of the
previous `PageViewModel` instance when a new one was created.

The fix is implemented in
`Microsoft.CmdPal.UI.ViewModels/ShellViewModel.cs` by ensuring that
`PageViewModel` instances are correctly disposed of when they are no
longer active.

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## PR Checklist

- [x] **Closes:** #40199
- [ ] **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

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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

1. Installed [Workspace Launcher for Visual Studio /
Code](https://github.com/tanchekwei/WorkspaceLauncherForVSCode)
2. Configured a global hotkey for the extension (e.g., Alt + Q).
3. Ran the following AHK script. Pressing F1 simulates pressing the
hotkey 10 times with a 200ms interval, followed by typing p to trigger a
search and observe how many times `GetItems` is invoked.

```ahk
#NoEnv
SetBatchLines, -1

; Press F1 to run the sequence
F1::
Loop, 10
{
    Send, !q
    Sleep, 200
}
Send, p
return
```

### Before the Fix (PowerToys v0.91.1)

- Pressing the hotkey 10 times then search will triggered `GetItems` 12
times.
- See video and log output below:


https://github.com/user-attachments/assets/3c7d59d6-2dda-4ab4-b230-2f2472c45776


```log
[2025-06-25 00:04:58.251] [VisualStudioCodePage.UpdateSearchText] Started
[2025-06-25 00:04:58.251] [VisualStudioCodePage.UpdateSearchText] SearchText: p
[2025-06-25 00:04:58.251] [WorkspaceFilter.Filter] Started
[2025-06-25 00:04:58.263] [WorkspaceFilter.Filter] Finished in 11ms
[2025-06-25 00:04:58.269] [VisualStudioCodePage.GetItems] Started
[2025-06-25 00:04:58.269] [VisualStudioCodePage.GetItems] Finished in 0ms
[2025-06-25 00:04:58.373] [VisualStudioCodePage.GetItems] Started
[2025-06-25 00:04:58.373] [VisualStudioCodePage.GetItems] Finished in 0ms
[2025-06-25 00:04:58.408] [VisualStudioCodePage.GetItems] Started
[2025-06-25 00:04:58.408] [VisualStudioCodePage.GetItems] Finished in 0ms
[2025-06-25 00:04:58.445] [VisualStudioCodePage.GetItems] Started
[2025-06-25 00:04:58.446] [VisualStudioCodePage.GetItems] Finished in 0ms
[2025-06-25 00:04:58.485] [VisualStudioCodePage.GetItems] Started
[2025-06-25 00:04:58.486] [VisualStudioCodePage.GetItems] Finished in 0ms
[2025-06-25 00:04:58.524] [VisualStudioCodePage.GetItems] Started
[2025-06-25 00:04:58.525] [VisualStudioCodePage.GetItems] Finished in 0ms
[2025-06-25 00:04:58.563] [VisualStudioCodePage.GetItems] Started
[2025-06-25 00:04:58.564] [VisualStudioCodePage.GetItems] Finished in 0ms
[2025-06-25 00:04:58.604] [VisualStudioCodePage.GetItems] Started
[2025-06-25 00:04:58.604] [VisualStudioCodePage.GetItems] Finished in 0ms
[2025-06-25 00:04:58.645] [VisualStudioCodePage.GetItems] Started
[2025-06-25 00:04:58.645] [VisualStudioCodePage.GetItems] Finished in 0ms
[2025-06-25 00:04:58.685] [VisualStudioCodePage.GetItems] Started
[2025-06-25 00:04:58.686] [VisualStudioCodePage.GetItems] Finished in 0ms
[2025-06-25 00:04:58.755] [VisualStudioCodePage.GetItems] Started
[2025-06-25 00:04:58.756] [VisualStudioCodePage.GetItems] Finished in 0ms
[2025-06-25 00:04:58.798] [VisualStudioCodePage.GetItems] Started
[2025-06-25 00:04:58.798] [VisualStudioCodePage.GetItems] Finished in 0ms
[2025-06-25 00:04:58.843] [VisualStudioCodePage.UpdateSearchText] Finished in 591ms
```

### After the Fix
- Pressing the hotkey 10 times then search will now triggers `GetItems`
only once.
- See updated video and log output:


https://github.com/user-attachments/assets/e38f3eb5-5353-44da-a9d9-ff83647eb6e9

```log

[2025-06-25 00:03:44.862] [VisualStudioCodePage.UpdateSearchText] Started
[2025-06-25 00:03:44.863] [VisualStudioCodePage.UpdateSearchText] SearchText: p
[2025-06-25 00:03:44.864] [WorkspaceFilter.Filter] Started
[2025-06-25 00:03:44.866] [WorkspaceFilter.Filter] Finished in 2ms
[2025-06-25 00:03:44.870] [VisualStudioCodePage.GetItems] Started
[2025-06-25 00:03:44.870] [VisualStudioCodePage.GetItems] Finished in 0ms
[2025-06-25 00:03:44.909] [VisualStudioCodePage.UpdateSearchText] Finished in 46ms
```

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

@@ -29,8 +29,30 @@ public partial class ShellViewModel(IServiceProvider _serviceProvider, TaskSched
     [ObservableProperty]
     public partial bool IsDetailsVisible { get; set; }
 
-    [ObservableProperty]
-    public partial PageViewModel CurrentPage { get; set; } = new LoadingPageViewModel(null, _scheduler);
+    private PageViewModel _currentPage = new LoadingPageViewModel(null, _scheduler);
+
+    public PageViewModel CurrentPage
+    {
+        get => _currentPage;
+        set
+        {
+            var oldValue = _currentPage;
+            if (SetProperty(ref _currentPage, value))
+            {
+                if (oldValue is IDisposable disposable)
+                {
+                    try
+                    {
+                        disposable.Dispose();
+                    }
+                    catch (Exception ex)
+                    {
+                        Logger.LogError(ex.ToString());
+                    }
+                }
+            }
+        }
+    }
 
     private MainListPage? _mainListPage;