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.Common/Helpers/DiagnosticsHelper.cs

@@ -0,0 +1,66 @@
+// 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.Runtime.InteropServices;
+
+namespace Microsoft.CmdPal.Common.Helpers;
+
+/// <summary>
+/// Provides utility methods for building diagnostic and error messages.
+/// </summary>
+public static class DiagnosticsHelper
+{
+    /// <summary>
+    /// Builds a comprehensive exception message with timestamp and detailed diagnostic information.
+    /// </summary>
+    /// <param name="exception">The exception that occurred.</param>
+    /// <param name="extensionHint">A hint about which extension caused the exception to help with debugging.</param>
+    /// <returns>A string containing the exception details, timestamp, and source information for diagnostic purposes.</returns>
+    public static string BuildExceptionMessage(Exception exception, string? extensionHint)
+    {
+        var locationHint = string.IsNullOrWhiteSpace(extensionHint) ? "application" : $"'{extensionHint}' extension";
+
+        // let's try to get a message from the exception or inferred it from the HRESULT
+        // to show at least something
+        var message = exception.Message;
+        if (string.IsNullOrWhiteSpace(message))
+        {
+            var temp = Marshal.GetExceptionForHR(exception.HResult)?.Message;
+            if (!string.IsNullOrWhiteSpace(temp))
+            {
+                message = temp + $" (inferred from HRESULT 0x{exception.HResult:X8})";
+            }
+        }
+
+        if (string.IsNullOrWhiteSpace(message))
+        {
+            message = "[No message available]";
+        }
+
+        // note: keep date time kind and format consistent with the log
+        return $"""
+                ============================================================
+                😢 An unexpected error occurred in the {locationHint}.
+
+                Summary:
+                  Message:    {message}
+                  Type:       {exception.GetType().FullName}
+                  Source:     {exception.Source ?? "N/A"}
+                  Time:       {DateTimeOffset.Now:yyyy-MM-dd HH:mm:ss.fffffff}
+                  HRESULT:    0x{exception.HResult:X8} ({exception.HResult})
+
+                Stack Trace:
+                {exception.StackTrace ?? "[No stack trace available]"}
+
+                ------------------ Full Exception Details ------------------
+                {exception}
+
+                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.
+                ============================================================
+
+                """;
+    }
+}

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);

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

@@ -410,8 +410,8 @@ public partial class TopLevelCommandManager : ObservableObject,
 
     void IPageContext.ShowException(Exception ex, string? extensionHint)
     {
-        var errorMessage = $"A bug occurred in {$"the \"{extensionHint}\"" ?? "an unknown's"} extension's code:\n{ex.Message}\n{ex.Source}\n{ex.StackTrace}\n\n";
-        CommandPaletteHost.Instance.Log(errorMessage);
+        var message = DiagnosticsHelper.BuildExceptionMessage(ex, extensionHint ?? "TopLevelCommandManager");
+        CommandPaletteHost.Instance.Log(message);
     }
 
     internal bool IsProviderActive(string id)