CmdPal: Harden ListViewModel fetch synchronization (#46429)

## Summary of the Pull Request

This PR improves fetching of list items in ListViewModel:
- Fixes _vmCache concurrency with copy-on-write cache publication.
- Preserves latest-fetch-wins behavior across overlapping RPC GetItems()
calls.
- Prevents stale or canceled fetches from publishing and makes them
abort promptly.
- Improves cancellation cleanup for abandoned item view models and
replaced token sources.
- Updates empty-state tracking to follow overlapping fetch activity
correctly.
- Reduces hot-path cache overhead by removing per-item cache locking and
full cache rebuilds.
- Adds guard against re-entry, to prevent situations like #46329:
- Defers ItemsChanged-triggered fetches raised during GetItems() until
the call unwinds;
- Uses a thread-local reentry guard so unrelated cross-thread fetches
are not delayed;
- Adds a regression test covering recursive GetItems() refresh behavior.
- Make sure we never invoke FetchItems on UI thread, and be loud in
debug when we are.

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## PR Checklist

- [x] Closes: #46331
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- [ ] **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
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- [ ] **Documentation updated:** If checked, please file a pull request
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and link it here: #xxx

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## Detailed Description of the Pull Request / Additional comments

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wherever possible, but list manual validation steps taken as well -->
## Validation Steps Performed

src/modules/cmdpal/Tests/Microsoft.CmdPal.UI.ViewModels.UnitTests/ListViewModelTests.cs

@@ -0,0 +1,89 @@
+// Copyright (c) Microsoft Corporation
+// The Microsoft Corporation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+using System;
+using System.Threading;
+using System.Threading.Tasks;
+using Microsoft.CommandPalette.Extensions;
+using Microsoft.CommandPalette.Extensions.Toolkit;
+using Microsoft.VisualStudio.TestTools.UnitTesting;
+
+namespace Microsoft.CmdPal.UI.ViewModels.UnitTests;
+
+[TestClass]
+public partial class ListViewModelTests
+{
+    private sealed partial class TestAppExtensionHost : AppExtensionHost
+    {
+        public override string? GetExtensionDisplayName() => "Test Host";
+    }
+
+    private sealed partial class RecursiveItemsChangedPage : ListPage
+    {
+        private int _getItemsCallCount;
+        private int _recursiveItemsChangedRaised;
+        private TaskCompletionSource<bool> _deferredFetchObserved = NewDeferredFetchObserved();
+
+        public int GetItemsCallCount => Volatile.Read(ref _getItemsCallCount);
+
+        public Task DeferredFetchObserved => _deferredFetchObserved.Task;
+
+        public bool RaiseItemsChangedDuringGetItems { get; set; }
+
+        public override IListItem[] GetItems()
+        {
+            var callCount = Interlocked.Increment(ref _getItemsCallCount);
+            if (callCount >= 2)
+            {
+                _deferredFetchObserved.TrySetResult(true);
+            }
+
+            if (RaiseItemsChangedDuringGetItems && Interlocked.Exchange(ref _recursiveItemsChangedRaised, 1) == 0)
+            {
+                RaiseItemsChanged(0);
+            }
+
+            return [new ListItem(new NoOpCommand() { Name = $"Item {callCount}" })];
+        }
+
+        public void PrepareRecursiveFetch()
+        {
+            Volatile.Write(ref _getItemsCallCount, 0);
+            Volatile.Write(ref _recursiveItemsChangedRaised, 0);
+            RaiseItemsChangedDuringGetItems = true;
+            _deferredFetchObserved = NewDeferredFetchObserved();
+        }
+
+        public void TriggerItemsChanged(int totalItems = 0) => RaiseItemsChanged(totalItems);
+
+        private static TaskCompletionSource<bool> NewDeferredFetchObserved() => new(TaskCreationOptions.RunContinuationsAsynchronously);
+    }
+
+    private static ListViewModel CreateViewModel(RecursiveItemsChangedPage page) =>
+        new(page, TaskScheduler.Default, new TestAppExtensionHost(), CommandProviderContext.Empty, DefaultContextMenuFactory.Instance);
+
+    [TestMethod]
+    public async Task RecursiveItemsChangedDuringGetItems_IsDeferredUntilGetItemsReturns()
+    {
+        var page = new RecursiveItemsChangedPage
+        {
+            Id = "list.page",
+            Name = "List Page",
+            Title = "List Page",
+        };
+
+        var viewModel = CreateViewModel(page);
+        viewModel.InitializeProperties();
+        page.PrepareRecursiveFetch();
+
+        page.TriggerItemsChanged();
+
+        var completed = await Task.WhenAny(page.DeferredFetchObserved, Task.Delay(TimeSpan.FromSeconds(2)));
+        Assert.AreSame(page.DeferredFetchObserved, completed);
+        Assert.AreEqual(2, page.GetItemsCallCount);
+
+        viewModel.SafeCleanup();
+        viewModel.Dispose();
+    }
+}