TextBox: Narrator announces "new line" when typing the first character

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### Describe the bug

With Narrator running, typing the **first** character into an empty `TextBox` makes Narrator
announce the character and then say **"new line"**. This happens only on the first character
(the empty→non-empty transition); every subsequent character echoes cleanly with no "new line"

### Why is this important?

Confusing Narrator experience on any first-character text entry. Hit in Windows OOBE text-entry pages.

### Steps to reproduce the bug

1. Turn Narrator on.
2. Focus an empty `TextBox`.
3. Type a single character.

### Actual behavior

Narrator announces the typed character and then "new line"

### Expected behavior

Narrator announces only the typed character.

### Screenshots

_No response_

### NuGet package version

1.6.250402001

### Windows version

Windows 11 (24H2): Build 26100

### Additional context

_No response_

Start by reproducing the issue in the TextBox with Narrator enabled, focusing on the empty-to-non-empty transition described in the steps. Trace the TextBox input and accessibility behavior to determine why the first character also produces “new line”; done means the character is announced without that extra announcement while later characters remain unchanged.

Written by the indexing model from the issue text.