CmdPal: Fix duplicate "Pin to..." context commands on top-level items (#46458)

Top-level commands on the home page showed duplicate pin context entries
— e.g., "Pin to Dock" appearing twice, or contradictory "Pin to Dock" +
"Unpin from dock" on the same command.

## Summary of the Pull Request

When the window is hidden while a sub-page is active,
`ShellViewModel.Receive(WindowHiddenMessage)` re-navigates to the root
page while `CurrentPage` still points to the sub-page.
`GetProviderContextForCommand` therefore returns the sub-page's
`ProviderContext` (which has `SupportsPinning = true`, `ProviderId =
<extension>`) for the new home-page `ListViewModel`.

With that wrong context, `UnsafeBuildAndInitMoreCommands` runs for each
`ListItemViewModel` wrapping a `TopLevelViewModel` and injects a second
set of pin commands — using the wrong provider's dock/top-level state —
on top of the ones `TopLevelViewModel.BuildContextMenu()` already
injected via `AddMoreCommandsToTopLevel` with the correct per-item
provider context.

**Changes:**

- **`ShellViewModel.cs` (root cause):** Move `isMainPage` evaluation
before `providerContext` is computed; use `CommandProviderContext.Empty`
when navigating to the root page, regardless of what `CurrentPage` is at
that moment.

  ```csharp
  var isMainPage = command == _rootPage;
  var providerContext = isMainPage
      ? CommandProviderContext.Empty
: _appHostService.GetProviderContextForCommand(message.Context,
CurrentPage.ProviderContext);
  ```

- **`CommandPaletteContextMenuFactory.cs` (defensive guard):** In
`UnsafeBuildAndInitMoreCommands`, bail early when the page context
supports pinning and `commandItem.Model.Unsafe is TopLevelViewModel`.
`BuildContextMenu()` on `TopLevelViewModel` already populates pin
commands via `AddMoreCommandsToTopLevel` with the item's own provider
context; adding them again here is always wrong regardless of how the
page context ended up. The `SupportsPinning` check is evaluated first to
skip the `.Unsafe` type-test in the common non-pinning case.

## PR Checklist

- [ ] **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

## Detailed Description of the Pull Request / Additional comments

The two fixes are complementary: the `ShellViewModel` change prevents
the wrong `ProviderContext` from ever reaching the home-page
`ListViewModel`; the `CommandPaletteContextMenuFactory` guard ensures
`TopLevelViewModel`-backed items are never double-processed even if some
other future code path sets the page context incorrectly.

The guard in
`CommandPaletteContextMenuFactory.UnsafeBuildAndInitMoreCommands` is
ordered so that `providerContext.SupportsPinning` (a cheap bool property
read) is evaluated before `commandItem.Model.Unsafe is
TopLevelViewModel`. This means the field access and type check are
skipped entirely for the common non-pinning case, addressing reviewer
feedback about unnecessary work on the hot path.

## Validation Steps Performed

Manually reproduced by opening CmdPal, navigating into a sub-page (e.g.,
"Search the Web"), closing the window, reopening, and verifying the
context menu for top-level commands shows exactly one "Pin to Dock" (or
"Unpin from dock") entry and at most one top-level pin action — no
duplicates or contradictory pairs.

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---------

Co-authored-by: copilot-swe-agent[bot] <[email protected]>
Co-authored-by: michaeljolley <[email protected]>