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/extensionsdk/Microsoft.CommandPalette.Extensions.Toolkit/KeyChordHelpers.cs

@@ -6,7 +6,7 @@ using Windows.System;
 
 namespace Microsoft.CommandPalette.Extensions.Toolkit;
 
-public partial class KeyChordHelpers
+public static partial class KeyChordHelpers
 {
     public static KeyChord FromModifiers(
         bool ctrl = false,
@@ -34,4 +34,28 @@ public partial class KeyChordHelpers
     {
         return FromModifiers(ctrl, alt, shift, win, (int)vkey, scanCode);
     }
+
+    public static string FormatForDebug(KeyChord value)
+    {
+        var result = string.Empty;
+
+        if (value.Modifiers.HasFlag(VirtualKeyModifiers.Control))
+        {
+            result += "Ctrl+";
+        }
+
+        if (value.Modifiers.HasFlag(VirtualKeyModifiers.Shift))
+        {
+            result += "Shift+";
+        }
+
+        if (value.Modifiers.HasFlag(VirtualKeyModifiers.Menu))
+        {
+            result += "Alt+";
+        }
+
+        result += (VirtualKey)value.Vkey;
+
+        return result;
+    }
 }