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

<!-- 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/Tests/Microsoft.CmdPal.Ext.WebSearch.UnitTests/SettingsManagerTests.cs

@@ -0,0 +1,48 @@
+// Copyright (c) Microsoft Corporation
+// 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.Threading.Tasks;
+
+using Microsoft.CmdPal.Ext.UnitTestBase;
+using Microsoft.CmdPal.Ext.WebSearch.Helpers;
+using Microsoft.CmdPal.Ext.WebSearch.Pages;
+using Microsoft.VisualStudio.TestTools.UnitTesting;
+
+namespace Microsoft.CmdPal.Ext.WebSearch.UnitTests;
+
+[TestClass]
+public class SettingsManagerTests : CommandPaletteUnitTestBase
+{
+    [TestMethod]
+    public async Task HistoryChangedEventIsRaisedWhenItemIsAdded()
+    {
+        // Setup
+        var settings = new MockSettingsInterface(historyItemCount: 5);
+        var page = new WebSearchListPage(settings);
+
+        var eventRaised = false;
+
+        try
+        {
+            settings.HistoryChanged += Handler;
+
+            // Act
+            settings.AddHistoryItem(new HistoryItem("test event", DateTime.UtcNow));
+            await Task.Delay(50);
+
+            // Assert
+            Assert.IsTrue(eventRaised, "Expected HistoryChanged to be raised when saving history.");
+        }
+        finally
+        {
+            settings.HistoryChanged -= Handler;
+            page.Dispose();
+        }
+
+        return;
+
+        void Handler(object s, EventArgs e) => eventRaised = true;
+    }
+}