CmdPal: Make settings and app state immutable (#46451)

## Summary
 
 This PR refactors CmdPal settings/state to be immutable end-to-end.
 
 ### Core changes
 - Convert model types to immutable records / init-only properties:
   - `SettingsModel`
   - `AppStateModel`
   - `ProviderSettings`
   - `DockSettings`
   - `RecentCommandsManager`
- supporting settings types (fallback/hotkey/alias/top-level
hotkey/history items, etc.)
- Replace mutable collections with immutable equivalents where
appropriate:
   - `ImmutableDictionary<,>`
   - `ImmutableList<>`
 - Move mutation flow to atomic service updates:
- `ISettingsService.UpdateSettings(Func<SettingsModel, SettingsModel>)`
   - `IAppStateService.UpdateState(Func<AppStateModel, AppStateModel>)`
- Update ViewModels/managers/services to use copy-on-write (`with`)
patterns instead of in-place
mutation.
- Update serialization context + tests for immutable model graph
compatibility.
 
 ## Why
 
Issue #46437 is caused by mutable shared state being updated from
different execution paths/threads,
leading to race-prone behavior during persistence/serialization.
 
By making settings/app state immutable and using atomic swap/update
patterns, we remove in-place
mutation and eliminate this class of concurrency bug.
 
 ## Validation
 
 - Built successfully:
   - `Microsoft.CmdPal.UI.ViewModels`
   - `Microsoft.CmdPal.UI`
   - `Microsoft.CmdPal.UI.ViewModels.UnitTests`
 - Updated unit tests for immutable update patterns.
 
 Fixes #46437

---------

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

src/modules/cmdpal/Microsoft.CmdPal.UI.ViewModels/FallbackSettingsViewModel.cs

@@ -12,7 +12,9 @@ namespace Microsoft.CmdPal.UI.ViewModels;
 public partial class FallbackSettingsViewModel : ObservableObject
 {
     private readonly ISettingsService _settingsService;
-    private readonly FallbackSettings _fallbackSettings;
+    private readonly ProviderSettingsViewModel _providerSettingsViewModel;
+
+    private FallbackSettings _fallbackSettings;
 
     public string DisplayName { get; private set; } = string.Empty;
 
@@ -27,15 +29,18 @@ public partial class FallbackSettingsViewModel : ObservableObject
         {
             if (value != _fallbackSettings.IsEnabled)
             {
-                _fallbackSettings.IsEnabled = value;
+                var newSettings = _fallbackSettings with { IsEnabled = value };
 
-                if (!_fallbackSettings.IsEnabled)
+                if (!newSettings.IsEnabled)
                 {
-                    _fallbackSettings.IncludeInGlobalResults = false;
+                    newSettings = newSettings with { IncludeInGlobalResults = false };
                 }
 
-                Save();
+                _fallbackSettings = newSettings;
+                _providerSettingsViewModel.UpdateFallbackSettings(Id, _fallbackSettings);
+
                 OnPropertyChanged(nameof(IsEnabled));
+                WeakReferenceMessenger.Default.Send<ReloadCommandsMessage>(new());
             }
         }
     }
@@ -47,15 +52,18 @@ public partial class FallbackSettingsViewModel : ObservableObject
         {
             if (value != _fallbackSettings.IncludeInGlobalResults)
             {
-                _fallbackSettings.IncludeInGlobalResults = value;
+                var newSettings = _fallbackSettings with { IncludeInGlobalResults = value };
 
-                if (!_fallbackSettings.IsEnabled)
+                if (!newSettings.IsEnabled)
                 {
-                    _fallbackSettings.IsEnabled = true;
+                    newSettings = newSettings with { IsEnabled = true };
                 }
 
-                Save();
+                _fallbackSettings = newSettings;
+                _providerSettingsViewModel.UpdateFallbackSettings(Id, _fallbackSettings);
+
                 OnPropertyChanged(nameof(IncludeInGlobalResults));
+                WeakReferenceMessenger.Default.Send<ReloadCommandsMessage>(new());
             }
         }
     }
@@ -67,6 +75,7 @@ public partial class FallbackSettingsViewModel : ObservableObject
     ISettingsService settingsService)
     {
         _settingsService = settingsService;
+        _providerSettingsViewModel = providerSettings;
         _fallbackSettings = fallbackSettings;
 
         Id = fallback.Id;
@@ -77,10 +86,4 @@ public partial class FallbackSettingsViewModel : ObservableObject
         Icon = new(fallback.InitialIcon);
         Icon.InitializeProperties();
     }
-
-    private void Save()
-    {
-        _settingsService.Save();
-        WeakReferenceMessenger.Default.Send<ReloadCommandsMessage>(new());
-    }
 }