"Open in..." cannot add MSIX-packaged apps via App Execution Aliases

### Short summary

Selecting an MSIX app's execution alias under `%LOCALAPPDATA%\Microsoft\WindowsApps` fails with "Cannot access file."

### Affected version or release

v1.0.26

### Installation context

GitHub Copilot App installed locally on Windows 11 Enterprise (ARM64). The issue affects local applications added through "Open in...".

### What happened?

I tried to add an MSIX-packaged desktop application to "Open in..." in the GitHub Copilot App.

In the "Choose application" file picker, I navigated to `%LOCALAPPDATA%\Microsoft\WindowsApps` and selected the app execution alias, `skimdown.exe`.

The file picker displayed the following error and did not allow me to select the alias:

> Cannot access file.

The corresponding message in the Japanese UI is `ファイルにアクセスできません。`

The alias itself works correctly when launched from the Windows Run dialog or a terminal. The target application is distributed only as an MSIX package, so there is no conventional unpackaged executable that I can select instead.

### Steps to reproduce

1. Launch GitHub Copilot App v1.0.26 on Windows.
2. Open "Open in..." and select "Choose application".
3. Navigate to `%LOCALAPPDATA%\Microsoft\WindowsApps`.
4. Select an MSIX App Execution Alias, such as `skimdown.exe`.
5. Click "Open".
6. Observe the "Cannot access file" error.

### Expected behavior

GitHub Copilot App should accept a valid Windows App Execution Alias and register the MSIX-packaged application as an "Open in..." target.

### Additional context

- OS: Windows 11 Enterprise, version 10.0.26200 (build 26200), ARM64
- GitHub Copilot App version: v1.0.26
- Example alias: `skimdown.exe`
- Alias location: `%LOCALAPPDATA%\Microsoft\WindowsApps`
- Target application distribution: MSIX only

Reproduce the failure in the “Open in...” application chooser using the `%LOCALAPPDATA%\Microsoft\WindowsApps\skimdown.exe` alias on Windows 11. No source file or test is named; trace the chooser’s file-selection validation and verify that a valid MSIX App Execution Alias can be accepted and registered as an “Open in...” target.

Written by the indexing model from the issue text.