NumberBox will sometimes not respond to input from the keyboard

### Describe the bug

Sometimes, the NumberBox control (`Microsoft.UI.Xaml.Controls.NumberBox`) will not properly take input from the keyboard. This happens after focusing the control, especially the first time the control is focused after it is given a value. When this bug occurs, the only way to get the control to accept input is either to un-focus and re-focus the control, or to use the arrow keys to move the cursor.

### Steps to reproduce the bug

I've found that this works best for applications that have been launched recently. It is more difficult (but not impossible) to reproduce a second or third time, even if the NumberBox is a different instance.

**NB**: This bug does not occur *every* time, but it is by no means rare.

1. Locate a NumberBox (e.g. in the WinUI 3 Gallery, winui3gallery://item/NumberBox)
2. If the NumberBox does not already have a value, enter a value
3. Un-focus the NumberBox
4. Re-focus the numberbox
5. Attempt to modify or delete the contents of the box

### Expected behavior

The highlighted value in the NumberBox should be overwritten by the number that was inputted to the box.

### Screenshots

_No response_

### NuGet package version

WinUI 3 - Windows App SDK 1.6.3: 1.6.241114003

### Windows version

Windows 11 (23H2): Build 22631

### Additional context

Bug has been present since at least WinAppSDK 1.6.240923002 (also tested in the latest, 1.6.241114003). I have been unable to reproduce it with WinUI 2.

Other factors considered:
* Appears on both .NET 8 and .NET 9
* Appears in both production and release
* Appears in several different apps from different publishers
* Appears across different machines running different Windows versions (Enterprise, Pro, etc)

Start by reproducing the issue in the WinUI 3 Gallery at winui3gallery://item/NumberBox, using Windows App SDK 1.6.3 on Windows 11 and following the listed focus and edit steps. Investigate the NumberBox control's keyboard and focus behavior; done means a selected value consistently accepts replacement or deletion immediately after refocusing, without requiring arrow keys or another focus change.

Written by the indexing model from the issue text.