CmdPal: Replace localized strings used as setting values in WebSearch extension with literals (#41331)

## Summary of the Pull Request

For WebSearch extension:

- Replaces localized string identifiers with invariant literal keys to
ensure stable and consistent setting values, avoiding issues when
switching cultures or if display strings change.
- Renames the `ShowHistory` property to `HistoryItemCount`.  
- Changes the type from `string` to `int` and centralizes parsing logic
in `SettingsManager`.
- Retains backward compatibility by preserving the legacy settings key
`"ShowHistory"` in `SettingsManager`.


<!-- Please review the items on the PR checklist before submitting-->
## PR Checklist

- [x] Closes: #40547 
- [ ] **Communication:** I've discussed this with core contributors
already. If the work hasn't been agreed, this work might be rejected
- [x] **Tests:** Added/updated and all pass
- [x] **Localization:** All end-user-facing strings can be localized
- [x] **Dev docs:** none
- [x] **New binaries:** none
- [x] **Documentation updated:** none

<!-- 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

src/modules/cmdpal/ext/Microsoft.CmdPal.Ext.WebSearch/Helpers/ISettingsInterface.cs

@@ -11,7 +11,7 @@ public interface ISettingsInterface
 {
     public bool GlobalIfURI { get; }
 
-    public string ShowHistory { get; }
+    public uint HistoryItemCount { get; }
 
     public List<ListItem> LoadHistory();