CmdPal: Improve page exception details for users (#41035)

## Summary of the Pull Request

Show timestamp, HRESULT (hex/decimal), and full Exception.ToString() in
the error message. Centralize message generation in a helper class for
consistency.

Example:

```
============================================================
😢 An unexpected error occurred in the 'Open' extension.

Summary:
  Message:    Operation is not valid due to the current state of the object. (inferred from HRESULT 0x80131509)
  Type:       System.Runtime.InteropServices.COMException
  Source:     WinRT.Runtime
  Time:       2025-08-07 15:54:20.4189499
  HRESULT:    0x80131509 (-2146233079)

Stack Trace:
   at WinRT.ExceptionHelpers.<ThrowExceptionForHR>g__Throw|38_0(Int32 hr)
   at ABI.Microsoft.CommandPalette.Extensions.IListPageMethods.GetItems(IObjectReference _obj)
   at Microsoft.CmdPal.Core.ViewModels.ListViewModel.FetchItems()
   at Microsoft.CmdPal.Core.ViewModels.ListViewModel.InitializeProperties()
   at Microsoft.CmdPal.Core.ViewModels.PageViewModel.InitializeAsync()

------------------ Full Exception Details ------------------
System.Runtime.InteropServices.COMException (0x80131509)
   at WinRT.ExceptionHelpers.<ThrowExceptionForHR>g__Throw|38_0(Int32 hr)
   at ABI.Microsoft.CommandPalette.Extensions.IListPageMethods.GetItems(IObjectReference _obj)
   at Microsoft.CmdPal.Core.ViewModels.ListViewModel.FetchItems()
   at Microsoft.CmdPal.Core.ViewModels.ListViewModel.InitializeProperties()
   at Microsoft.CmdPal.Core.ViewModels.PageViewModel.InitializeAsync()

ℹ️ If you need further assistance, please include this information in your support request.
ℹ️ Before sending, take a quick look to make sure it doesn't contain any personal or sensitive information.
============================================================

```

<!-- Please review the items on the PR checklist before submitting-->
## PR Checklist

- [x] Closes: #41034
- [ ] **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

<!-- 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

## Validation Steps Performed

I crashed an extension on purpose and read the message.

src/modules/cmdpal/Microsoft.CmdPal.Core.ViewModels/PageViewModel.cs

@@ -5,6 +5,7 @@
 using System.Collections.ObjectModel;
 using CommunityToolkit.Mvvm.ComponentModel;
 using CommunityToolkit.Mvvm.Input;
+using Microsoft.CmdPal.Common.Helpers;
 using Microsoft.CmdPal.Core.ViewModels.Models;
 using Microsoft.CommandPalette.Extensions;
 
@@ -223,9 +224,10 @@ public partial class PageViewModel : ExtensionObjectViewModel, IPageContext
         extensionHint ??= ExtensionHost.GetExtensionDisplayName() ?? Title;
         Task.Factory.StartNew(
             () =>
-        {
-            ErrorMessage += $"A bug occurred in {$"the \"{extensionHint}\"" ?? "an unknown's"} extension's code:\n{ex.Message}\n{ex.Source}\n{ex.StackTrace}\n\n";
-        },
+            {
+                var message = DiagnosticsHelper.BuildExceptionMessage(ex, extensionHint);
+                ErrorMessage += message;
+            },
             CancellationToken.None,
             TaskCreationOptions.None,
             Scheduler);