Using ENTER in the application's sysmenu is triggerring in-app actions

### Describe the bug

Pressing the ENTER key in one of the entry of the window's system menu is also triggering the action associated to the focused XAML element.

### Steps to reproduce the bug

1. Launch the WinUI 3 gallery application
2. Open the "Button" page
3. Focus one of the sample button
4. Press ALT+SPACE to open the window's system menu
5. Highlight the maximize entry
6. Press ENTER

### Actual behavior

The window is maximized _and_ the button is pressed.
We can see the "You clicked: Button1" message in the sample output.

### Expected behavior

The window is maximized *and* the button is *not* pressed.

### Screenshots


### NuGet package version

WinUI 3 - Windows App SDK 1.7.3: 1.7.250606001

### Windows version

_No response_

### Additional context

_No response_

Reproduce the issue in the WinUI 3 Gallery application's Button page by focusing a sample button, opening the window system menu with Alt+Space, and activating Maximize with Enter. Trace the relevant window system-menu and XAML keyboard handling entry points; done means the window maximizes without also invoking the focused button action.

Written by the indexing model from the issue text.