[CmdPal][UnitTests] Add/Migrate unit test for WebSearch and Shell extension (#41272)

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- [x] Closes: #41241 #41242
- [x] **Communication:** I've discussed this with core contributors
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- [x] **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
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pipeline](https://github.com/microsoft/PowerToys/blob/main/.pipelines/ci/templates/build-powertoys-steps.yml)
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pipeline](https://github.com/microsoft/PowerToys/blob/main/.pipelines/release.yml)
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and link it here: #xxx

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Co-authored-by: Yu Leng <yuleng@microsoft.com>

src/modules/cmdpal/ext/Microsoft.CmdPal.Ext.Shell/Helpers/SettingsManager.cs

@@ -9,7 +9,7 @@ using Microsoft.CommandPalette.Extensions.Toolkit;
 
 namespace Microsoft.CmdPal.Ext.Shell.Helpers;
 
-public class SettingsManager : JsonSettingsManager
+public class SettingsManager : JsonSettingsManager, ISettingsInterface
 {
     private static readonly string _namespace = "shell";