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`.


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## 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

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or any additional comments/features here -->
## Detailed Description of the Pull Request / Additional comments

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wherever possible, but list manual validation steps taken as well -->
## Validation Steps Performed

src/modules/cmdpal/Tests/Microsoft.CmdPal.Ext.WebSearch.UnitTests/QueryTests.cs

@@ -54,7 +54,7 @@ public class QueryTests : CommandPaletteUnitTestBase
             new HistoryItem("another search", DateTime.Parse("2024-01-02 13:00:00", CultureInfo.CurrentCulture)),
         };
 
-        var settings = new MockSettingsInterface(mockHistory: mockHistoryItems, showHistory: "5");
+        var settings = new MockSettingsInterface(mockHistory: mockHistoryItems, historyItemCount: 5);
 
         var page = new WebSearchListPage(settings);
 
@@ -89,7 +89,7 @@ public class QueryTests : CommandPaletteUnitTestBase
             new HistoryItem("another search4", DateTime.Parse("2024-01-05 13:00:00", CultureInfo.CurrentCulture)),
         };
 
-        var settings = new MockSettingsInterface(mockHistory: mockHistoryItems, showHistory: "5");
+        var settings = new MockSettingsInterface(mockHistory: mockHistoryItems, historyItemCount: 5);
 
         var page = new WebSearchListPage(settings);
 
@@ -122,7 +122,7 @@ public class QueryTests : CommandPaletteUnitTestBase
             new HistoryItem("another search5", DateTime.Parse("2024-01-06 13:00:00", CultureInfo.CurrentCulture)),
         };
 
-        var settings = new MockSettingsInterface(mockHistory: mockHistoryItems, showHistory: "None");
+        var settings = new MockSettingsInterface(mockHistory: mockHistoryItems, historyItemCount: 0);
 
         var page = new WebSearchListPage(settings);