CmdPal: Handle exceptions when enqueuing callbacks to UI thread in IconCacheService (#40716)

## Summary of the Pull Request

Handle exceptions thrown in TryEnqueue callbacks so they don’t crash the
app (as they cannot be caught by the global exception handler). Any
exceptions are now returned to the caller for handling. Additionally, a
failure to enqueue the operation onto the dispatcher will also result in
an exception.

This is not a breaking change, as exceptions only propagate within the
class and do not affect external callers.

Ref: #38260

## PR Checklist

- [ ] **Closes:** #38260 
- [ ] **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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## Detailed Description of the Pull Request / Additional comments

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## Validation Steps Performed

src/modules/cmdpal/Microsoft.CmdPal.UI/Helpers/IconCacheService.cs

@@ -34,9 +34,9 @@ public sealed class IconCacheService(DispatcherQueue dispatcherQueue)
                 {
                     return await StreamToIconSource(icon.Data.Unsafe!);
                 }
-                catch
+                catch (Exception ex)
                 {
-                    Debug.WriteLine("Failed to load icon from stream");
+                    Debug.WriteLine("Failed to load icon from stream: " + ex);
                 }
             }
         }
@@ -63,17 +63,37 @@ public sealed class IconCacheService(DispatcherQueue dispatcherQueue)
     {
         // Return the bitmap image via TaskCompletionSource. Using WCT's EnqueueAsync does not suffice here, since if
         // we're already on the thread of the DispatcherQueue then it just directly calls the function, with no async involved.
-        var completionSource = new TaskCompletionSource<BitmapImage>();
-        dispatcherQueue.TryEnqueue(async () =>
+        return await TryEnqueueAsync(dispatcherQueue, async () =>
         {
             using var bitmapStream = await iconStreamRef.OpenReadAsync();
             var itemImage = new BitmapImage();
             await itemImage.SetSourceAsync(bitmapStream);
-            completionSource.TrySetResult(itemImage);
+            return itemImage;
+        });
+    }
+
+    private static Task<T> TryEnqueueAsync<T>(DispatcherQueue dispatcher, Func<Task<T>> function)
+    {
+        var completionSource = new TaskCompletionSource<T>();
+
+        var enqueued = dispatcher.TryEnqueue(DispatcherQueuePriority.Normal, async void () =>
+        {
+            try
+            {
+                var result = await function();
+                completionSource.SetResult(result);
+            }
+            catch (Exception ex)
+            {
+                completionSource.SetException(ex);
+            }
         });
 
-        var bitmapImage = await completionSource.Task;
+        if (!enqueued)
+        {
+            completionSource.SetException(new InvalidOperationException("Failed to enqueue the operation on the UI dispatcher"));
+        }
 
-        return bitmapImage;
+        return completionSource.Task;
     }
 }