Add Desktop Peek to Shortcut Guide Windows shortcuts (#49638)

<!-- 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

Shortcut Guide was missing the Windows desktop peek shortcut (`Win + ,`)
from the Windows shell shortcuts it displays. This update adds the
missing entry and pins it with a focused manifest test.

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## PR Checklist

- [ ] **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

<!-- 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

- **Shortcut manifest**
- Adds `Peek at desktop temporarily` to the Windows shell manifest in
the `Windows key` section.
- Models the shortcut as `Win + ,`, matching the OS behavior Shortcut
Guide should surface.

- **Regression coverage**
- Adds a focused unit test that deserializes
`+WindowsNT.Shell.en-US.yml` and asserts the Desktop Peek entry is
present with the expected shortcut payload.

```yml
- Name: Peek at desktop temporarily
  Shortcut:
    - Win: true
      Ctrl: false
      Shift: false
      Alt: false
      Keys:
        - ","
```

<!-- Describe how you validated the behavior. Add automated tests
wherever possible, but list manual validation steps taken as well -->
## Validation Steps Performed

- Parsed the updated `+WindowsNT.Shell.en-US.yml` manifest and verified
the new entry is present in the `Windows key` section.
- Added a manifest-focused unit test covering the new shortcut entry.

<!-- START COPILOT CODING AGENT SUFFIX -->

- Fixes #49458

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>

src/modules/ShortcutGuide/ShortcutGuide.Ui/Assets/ShortcutGuide/Manifests/+WindowsNT.Shell.en-US.yml

@@ -282,6 +282,14 @@ Shortcuts:
             Keys:
               - "Space"
         Description: To previous option
+      - Name: Peek at desktop temporarily
+        Shortcut:
+          - Win: true
+            Ctrl: false
+            Shift: false
+            Alt: false
+            Keys:
+              - ","
       - Name: Display/Hide desktop
         Shortcut:
           - Win: true