CmdPal: Update WebSearch extension history immediately (#41398)

## Summary of the Pull Request

This PR ensures that the list of recent searches in the Web Search
extension is updated immediately after a new item is added or when
settings controlling the number of items are changed.

- Refactors the Web Search extension history to keep it in memory after
being loaded at startup
- Adds an event to notify subscribers when the history changes  
- Implements `IDisposable` to ensure that `WebSearchListPage`
unsubscribes from the event
- Moves responsibility for creating all list items to single class
(`WebSearchListPage`)
- Updated unit tests
- 
## PR Checklist

- [x] Closes: #40548
- [ ] **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:** nothing
- [x] **New binaries:** none
- [x] **Documentation updated:** nope

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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/ext/Microsoft.CmdPal.Ext.WebSearch/Helpers/ISettingsInterface.cs

@@ -2,6 +2,7 @@
 // The Microsoft Corporation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+using System;
 using System.Collections.Generic;
 using Microsoft.CommandPalette.Extensions.Toolkit;
 
@@ -9,11 +10,13 @@ namespace Microsoft.CmdPal.Ext.WebSearch.Helpers;
 
 public interface ISettingsInterface
 {
+    event EventHandler? HistoryChanged;
+
     public bool GlobalIfURI { get; }
 
-    public uint HistoryItemCount { get; }
+    public int HistoryItemCount { get; }
 
-    public List<ListItem> LoadHistory();
+    public IReadOnlyList<HistoryItem> HistoryItems { get; }
 
-    public void SaveHistory(HistoryItem historyItem);
+    public void AddHistoryItem(HistoryItem historyItem);
 }