NumberBox: screen reader reads "new line" when mouse clicks arrows

### Describe the bug

With Narrator enabled, clicking on the arrow buttons for the `NumberBox` control results in the screen reader reading out "new line". The announcement should instead reflect the updated value. Issue repros in WinUI 2 Gallery > [NumberBox page](winui2gallery://item/NumberBox)

### Why is this important?

Screen readers are useful for limited-visibility users. A scenario like this is important because this impacts users who use a screen reader and a mouse.

### Steps to reproduce the bug

1. In WinUI 2 Gallery, go to [NumberBox page](winui2gallery://item/NumberBox)
2. Turn on the narrator using 'CTRL+Win+Enter'
3. Change the value by up and down arrow buttons

### Actual behavior

"New line" read out

### Expected behavior

The updated value should be announced

### Screenshots

https://github.com/user-attachments/assets/2996eca8-6829-45f1-bc49-484f907a5bab

### NuGet package version

None

### Windows version

_No response_

### Additional context

Originally filed in https://github.com/microsoft/terminal/issues/19693

Start with the WinUI 2 Gallery NumberBox page and reproduce the issue using Narrator and the arrow buttons. Trace the NumberBox accessibility behavior that produces the “new line” announcement, then verify that changing the value announces the updated value instead. Confirm the result with the documented reproduction steps.

Written by the indexing model from the issue text.