CmdPal: Add drag & drop support (#44165)

## Summary of the Pull Request

This PR adds basic drag-and-drop support for items in list and grid
views.

It introduces two new properties on `ListItem`, backed by
`IExtendedAttributesProvider`: `DataPackage` and `DataPackageView`.
These properties are mutually exclusive.
`DataPackage` serves as a convenience property allowing the item to
retain the underlying object without risk of losing it. Across the
extension boundary, only the immutable `DataPackageView` snapshot is
transferred. When `DataPackage` is set, `DataPackageView` is derived
from it.

This PR includes initial concrete drag-and-drop implementations for:
- File Indexer  
- Clipboard History  

**Todo / Missing pieces** 
- [x] Extend `DataPackage` support to top-level command items, enabling
scenarios such as index fallback ~
- [x] Provide automatic drag-and-drop for unconfigured list items (e.g.,
copying title and subtitle as text)
- [x] Keep CmdPal open
- [ ] ~Clipboard commands (since we have the DataPackage...)~
- [ ] ~Improve logging~

## Pictures? Moving ones!


https://github.com/user-attachments/assets/13eb9a71-e760-43ea-8c2d-cd41cf377905




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## PR Checklist

- [x] Closes: #38289 
<!-- - [ ] Closes: #yyy (add separate lines for additional resolved
issues) -->
- [ ] **Communication:** I've discussed this with core contributors
already. If the work hasn't been agreed, this work might be rejected
- [ ] **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
signing](https://github.com/microsoft/PowerToys/blob/main/.pipelines/ESRPSigning_core.json)
for new binaries
- [ ] [WXS for
installer](https://github.com/microsoft/PowerToys/blob/main/installer/PowerToysSetup/Product.wxs)
for new binaries and localization folder
- [ ] [YML for CI
pipeline](https://github.com/microsoft/PowerToys/blob/main/.pipelines/ci/templates/build-powertoys-steps.yml)
for new test projects
- [ ] [YML for signed
pipeline](https://github.com/microsoft/PowerToys/blob/main/.pipelines/release.yml)
- [ ] **Documentation updated:** If checked, please file a pull request
on [our docs
repo](https://github.com/MicrosoftDocs/windows-uwp/tree/docs/hub/powertoys)
and link it here: #xxx

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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/Microsoft.CmdPal.UI.ViewModels/TopLevelViewModel.cs

@@ -18,7 +18,7 @@ using WyHash;
 
 namespace Microsoft.CmdPal.UI.ViewModels;
 
-public sealed partial class TopLevelViewModel : ObservableObject, IListItem
+public sealed partial class TopLevelViewModel : ObservableObject, IListItem, IExtendedAttributesProvider
 {
     private readonly SettingsModel _settings;
     private readonly ProviderSettings _providerSettings;
@@ -232,6 +232,13 @@ public sealed partial class TopLevelViewModel : ObservableObject, IListItem
             {
                 UpdateInitialIcon();
             }
+            else if (e.PropertyName == nameof(CommandItem.DataPackage))
+            {
+                DoOnUiThread(() =>
+                {
+                    OnPropertyChanged(nameof(CommandItem.DataPackage));
+                });
+            }
         }
     }
 
@@ -394,4 +401,12 @@ public sealed partial class TopLevelViewModel : ObservableObject, IListItem
     {
         return $"{nameof(TopLevelViewModel)}: {Id} ({Title}) - display: {DisplayTitle} - fallback: {IsFallback} - enabled: {IsEnabled}";
     }
+
+    public IDictionary<string, object?> GetProperties()
+    {
+        return new Dictionary<string, object?>
+        {
+            [WellKnownExtensionAttributes.DataPackage] = _commandItemViewModel?.DataPackage,
+        };
+    }
 }