[WinUI][dotnet8] Down Arrow does not call KeyDown event with cursor at '0' position in Entry

### Description

When I position the cursor at the beginning of an Entry, when I press the down key, no event is triggered, nor is Key Down triggered. However, when I click the up arrow, with the cursor in the same position, KeyDown is triggered. Does anyone else have this problem?

### Steps to Reproduce

In an Entry, implement a KeyDown event to trigger when any key is pressed;
Place the cursor at position 0 in that Entry, before any character;
Press the up arrow key -> The KeyDown Event is called;
Press the down arrow key -> The Event is NOT called;
Move the cursor 1 character to the right (position = 1).
Press the down arrow key -> Here, the event is called;

### Link to public reproduction project repository

_No response_

### Version with bug

8.0.14 SR3.1

### Is this a regression from previous behavior?

Yes, this used to work in .NET MAUI

### Last version that worked well

8.0.14 SR3.1

### Affected platforms

Windows

### Affected platform versions

_No response_

### Did you find any workaround?

_No response_

### Relevant log output

```shell

```

Start by reproducing the issue on Windows with a .NET MAUI Entry and a KeyDown handler, using the cursor positions and arrow keys described in the report. Then trace the Windows Entry keyboard-input path and compare Down Arrow behavior at positions 0 and 1. Done means the handler is called consistently at position 0 without breaking the existing Up Arrow behavior.

Written by the indexing model from the issue text.