CmdPal: Fix CmdPal command bar not refreshing on back navigation (#47126)

## Summary

Fixes #46810 — When navigating backward in CmdPal (Esc/Backspace), the
bottom command bar retained stale commands from the previous page until
the user changed selection.

## Root Cause

In `ListPage.xaml.cs` `OnNavigatedTo()`, the back-navigation path sets
selection using `SuppressSelectionChangedScope()`, which prevents
`Items_SelectionChanged` from firing. This means `PushSelectionToVm()`
is never called, so `UpdateCommandBarMessage` is never sent to
`CommandBarViewModel`, and the command bar displays stale state.

## Fix

Added `PushSelectionToVm()` after the selection restoration block inside
the back-navigation dispatcher callback. This is safe because:
- `PushSelectionToVm()` is idempotent (guards with
`ReferenceEquals(_lastPushedToVm, li)`)
- Handles both selected items (pushes to VM) and no selection (sends
null)
- Triggers `UpdateCommandBarMessage` which refreshes the command bar

## Validation

### Manual Test Steps

| Scenario | Expected |
|----------|----------|
| **Esc navigation** — Navigate into nested page, press Esc | Bottom bar
commands update immediately to parent page |
| **Backspace navigation** — Navigate into nested page, press Backspace
| Bottom bar commands update immediately to parent page |
| **Forward navigation** — Navigate forward into a page | Commands
update (no regression) |
| **Selection change** — Change selection on any page | Commands update
(no regression) |
| **No flicker** — Navigate back and observe command bar | Single clean
update, no double-fire |
| **Empty page** — Navigate back to page with no selectable items |
Graceful handling, no crash |

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

src/modules/cmdpal/Microsoft.CmdPal.UI/ExtViews/ListPage.xaml.cs

@@ -106,6 +106,9 @@ public sealed partial class ListPage : Page,
                         }
                     }
                 }
+
+                // Ensure the command bar refreshes with the restored page's commands.
+                PushSelectionToVm();
             });
         }