ReunionCaptionControlsWindow ButtonHoverBackgroundColor does not dismiss when there is another window in front of it

### Describe the bug

Normal win32 window caption button / uwp window does not have this issue.

### Steps to reproduce the bug

1. Open two winui 3 gallery
2. Place one in front of another, align the right edge, but stack them with a space that's less than the title bar height
3. Make the front window active
4. Move your mouse from the back window's caption button (so it goes into hover state), to the front window. Notice the button in the back window does not go back to normal state.

### Expected behavior

_No response_

### Screenshots


### NuGet package version

WinUI 3 - Windows App SDK 1.6.5: 1.6.250205002

### Windows version

Windows 11 (24H2): Build 26100

### Additional context

_No response_

Reproduce the issue in two WinUI 3 Gallery windows using the alignment and mouse-movement steps described. Trace the ReunionCaptionControlsWindow caption-button hover handling, then verify that the back window's button returns to its normal state when the pointer moves to the front window.

Written by the indexing model from the issue text.