CmdPal: Prevent crash on duplicate keybindings; simplify matching (#41714)

## Summary of the Pull Request

Handles duplicate keybindings by using the first occurrence and ignoring
the rest (in `ContextMenuViewModel.Keybindings` and
`IContextMenuContext.Keybindings`).

Replaces LINQ with direct iteration for clarity.

Simplifies `CheckKeybinding` by removing redundant null checks and
clarifying the key-to-binding matching logic, improving both readability
and efficiency.

Add a new method to `KeyChordHelpers.FormatForDebug` that formats
KeyChord as string to help debugging.

Makes `KeyChordHelpers` class a static class.

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## PR Checklist

- [x] Closes: #41712
- [ ] **Communication:** I've discussed this with core contributors
already. If the work hasn't been agreed, this work might be rejected
- [x] **Tests:** all pass
- [x] **Localization:** All end-user-facing strings can be localized
- [x] **Dev docs:** Added/updated
- [x] **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)
- [x] **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

Validated using a custom extension that has a duplicate item in the
context menu.

src/modules/cmdpal/ext/SamplePagesExtension/EvilSamplesPage.cs

@@ -219,7 +219,12 @@ public partial class EvilSamplesPage : ListPage
                 }
             ],
         },
-
+        new ListItem(new EvilDuplicateRequestedShortcut())
+        {
+            Title = "Evil keyboard shortcuts",
+            Subtitle = "Two commands with the same shortcut and more...",
+            Icon = new IconInfo("\uE765"),
+        }
     ];
 
     public EvilSamplesPage()
@@ -414,3 +419,42 @@ internal sealed partial class EvilFastUpdatesPage : DynamicListPage
         }
     }
 }
+
+[System.Diagnostics.CodeAnalysis.SuppressMessage("StyleCop.CSharp.MaintainabilityRules", "SA1402:File may only contain a single type", Justification = "Sample code")]
+internal sealed partial class EvilDuplicateRequestedShortcut : ListPage
+{
+    private readonly IListItem[] _items =
+    [
+        new ListItem(new NoOpCommand())
+        {
+            Title = "I'm evil!",
+            Subtitle = "I have multiple commands sharing the same keyboard shortcut",
+            MoreCommands = [
+                new CommandContextItem(new AnonymousCommand(() => new ToastStatusMessage("Me too executed").Show())
+                {
+                    Result = CommandResult.KeepOpen(),
+                })
+                {
+                    Title = "Me too",
+                    RequestedShortcut = KeyChordHelpers.FromModifiers(ctrl: true, vkey: VirtualKey.Number1),
+                },
+                new CommandContextItem(new AnonymousCommand(() => new ToastStatusMessage("Me three executed").Show())
+                {
+                    Result = CommandResult.KeepOpen(),
+                })
+                {
+                    Title = "Me three",
+                    RequestedShortcut = KeyChordHelpers.FromModifiers(ctrl: true, vkey: VirtualKey.Number1),
+                },
+            ],
+        },
+    ];
+
+    public override IListItem[] GetItems() => _items;
+
+    public EvilDuplicateRequestedShortcut()
+    {
+        Icon = new IconInfo(string.Empty);
+        Name = "Open";
+    }
+}