CmdPal: Prevent unsynchornized access to More commands (#46020)

## Summary of the Pull Request

This PR fixes a crash caused by unsynchronized access to a context menu:

- Fixes `System.ArgumentException: Destination array was not long
enough` crash in `CommandItemViewModel.AllCommands` caused by
`List<T>.AddRange()` racing with background `BuildAndInitMoreCommands`
mutations
- Replaces mutable `List<T>` public surfaces with immutable array
snapshots protected by a `Lock`; writers hold the lock, mutate the
backing list, then atomically publish new snapshots via `volatile`
fields that readers access lock-free
- Applies the same snapshot pattern to `ContentPageViewModel`, using a
bundled `CommandSnapshot` object for atomic publication (since
`PrimaryCommand` drives command invocation there, not just UI hints)
- Narrows `IContextMenuContext.MoreCommands` and `AllCommands` from
`List<T>`/`IEnumerable<T>` to `IReadOnlyList<T>` to prevent consumers
from casting back and mutating
- Moves `SafeCleanup()` calls outside locks in cleanup paths to avoid
holding the lock during cross-process RPC calls

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## PR Checklist

- [x] Closes: #45975 
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issues) -->
- [ ] **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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- [ ] **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

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

@@ -0,0 +1,79 @@
+// 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.Tasks;
+using Microsoft.CmdPal.UI.ViewModels.Models;
+using Microsoft.CommandPalette.Extensions.Toolkit;
+using Microsoft.VisualStudio.TestTools.UnitTesting;
+
+namespace Microsoft.CmdPal.UI.ViewModels.UnitTests;
+
+[TestClass]
+public class CommandItemViewModelTests
+{
+    private sealed class TestPageContext : IPageContext
+    {
+        public TaskScheduler Scheduler => TaskScheduler.Default;
+
+        public ICommandProviderContext ProviderContext => CommandProviderContext.Empty;
+
+        public void ShowException(Exception ex, string? extensionHint = null)
+        {
+            throw new AssertFailedException($"Unexpected exception from view model: {ex}");
+        }
+    }
+
+    [TestMethod]
+    public void MoreCommandsAndAllCommands_ReturnSnapshots()
+    {
+        // The public getters should return cached read-only snapshots, so
+        // repeated reads don't allocate a new list when the backing data hasn't
+        // changed.
+        var pageContext = new TestPageContext();
+        var item = new CommandItem(new NoOpCommand { Name = "Primary" })
+        {
+            Title = "Primary",
+            MoreCommands =
+            [
+                new CommandContextItem(new NoOpCommand { Name = "Secondary" }),
+            ],
+        };
+
+        var viewModel = new CommandItemViewModel(new(item), new(pageContext), DefaultContextMenuFactory.Instance);
+        viewModel.SlowInitializeProperties();
+
+        var moreCommands = viewModel.MoreCommands;
+        var allCommands = viewModel.AllCommands;
+
+        Assert.AreSame(moreCommands, viewModel.MoreCommands);
+        Assert.AreSame(allCommands, viewModel.AllCommands);
+        Assert.AreEqual(1, moreCommands.Count);
+        Assert.AreEqual(2, allCommands.Count);
+    }
+
+    [TestMethod]
+    public void SecondaryCommand_IgnoresLeadingSeparators()
+    {
+        // SecondaryCommand/HasMoreCommands should be derived from the first actual command item,
+        // not from the raw first entry in MoreCommands.
+        var pageContext = new TestPageContext();
+        var item = new CommandItem(new NoOpCommand { Name = "Primary" })
+        {
+            Title = "Primary",
+            MoreCommands =
+            [
+                new Separator("Group"),
+                new CommandContextItem(new NoOpCommand { Name = "Secondary" }),
+            ],
+        };
+
+        var viewModel = new CommandItemViewModel(new(item), new(pageContext), DefaultContextMenuFactory.Instance);
+        viewModel.SlowInitializeProperties();
+
+        Assert.IsTrue(viewModel.HasMoreCommands);
+        Assert.IsNotNull(viewModel.SecondaryCommand);
+        Assert.AreEqual("Secondary", viewModel.SecondaryCommand.Name);
+    }
+}

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

@@ -0,0 +1,104 @@
+// 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.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 ContentPageViewModelTests
+{
+    private sealed partial class TestAppExtensionHost : AppExtensionHost
+    {
+        public override string? GetExtensionDisplayName() => "Test Host";
+    }
+
+    private sealed partial class TestContentPage : ContentPage
+    {
+        public override IContent[] GetContent() => [];
+    }
+
+    private static CommandContextItem Command(string name) => new(new NoOpCommand { Name = name });
+
+    private static ContentPageViewModel CreateViewModel(TestContentPage page) =>
+        new(page, TaskScheduler.Default, new TestAppExtensionHost(), CommandProviderContext.Empty);
+
+    [TestMethod]
+    public void AllCommandsAndMoreCommands_ReturnCachedSnapshots()
+    {
+        // Content pages should expose stable snapshots, not the live Commands
+        // list, so repeated reads don't allocate and callers can't observe
+        // in-place list mutations.
+        var page = new TestContentPage
+        {
+            Id = "content.page",
+            Name = "Content Page",
+            Title = "Content Page",
+            Commands =
+            [
+                Command("Primary"),
+                Command("Secondary"),
+            ],
+        };
+
+        var viewModel = CreateViewModel(page);
+        viewModel.InitializeProperties();
+
+        var allCommands = viewModel.AllCommands;
+        var moreCommands = viewModel.MoreCommands;
+
+        Assert.AreSame(allCommands, viewModel.AllCommands);
+        Assert.AreSame(moreCommands, viewModel.MoreCommands);
+        Assert.AreEqual(2, allCommands.Count);
+        Assert.AreEqual(1, moreCommands.Count);
+        Assert.AreEqual("Primary", viewModel.PrimaryCommand?.Name);
+        Assert.AreEqual("Secondary", viewModel.SecondaryCommand?.Name);
+    }
+
+    [TestMethod]
+    public void CommandsUpdate_RefreshesSnapshotsConsistently()
+    {
+        // Updating the model commands should swap in a new coherent snapshot.
+        // The old snapshots stay intact, and the new cached values agree on
+        // counts, primary/secondary commands, and "has more" state.
+        var page = new TestContentPage
+        {
+            Id = "content.page",
+            Name = "Content Page",
+            Title = "Content Page",
+            Commands =
+            [
+                Command("Primary"),
+                Command("Secondary"),
+            ],
+        };
+
+        var viewModel = CreateViewModel(page);
+        viewModel.InitializeProperties();
+
+        var oldAllCommands = viewModel.AllCommands;
+        var oldMoreCommands = viewModel.MoreCommands;
+
+        page.Commands =
+        [
+            Command("Updated Primary"),
+            new Separator("Group"),
+            Command("Updated Secondary"),
+        ];
+
+        Assert.AreEqual(2, oldAllCommands.Count);
+        Assert.AreEqual(1, oldMoreCommands.Count);
+
+        Assert.AreEqual(3, viewModel.AllCommands.Count);
+        Assert.AreEqual(2, viewModel.MoreCommands.Count);
+        Assert.IsTrue(viewModel.HasCommands);
+        Assert.IsTrue(viewModel.HasMoreCommands);
+        Assert.AreEqual("Updated Primary", viewModel.PrimaryCommand?.Name);
+        Assert.AreEqual("Updated Secondary", viewModel.SecondaryCommand?.Name);
+        Assert.AreEqual("Updated Secondary", viewModel.SecondaryCommandName);
+    }
+}