UIA: spurious text selection changed event on rapid text deletion

### Windows Terminal version (or Windows build number)

Conhost (all versions with UIA)

### Other Software

NVDA.

### Steps to reproduce

1. Open a conhost in which NVDA relies on caret events to detect movement (currently not enabled in consoles)
2. quickly delete text with backspace.

### Expected Behavior

NVDA reads all deleted characters, then stays silent on all subsequent presses of backspace.

### Actual Behavior

The last character of the prompt (`>`) is read, implying its deletion.

Start by reproducing rapid backspace deletion in conhost with NVDA and observe the UIA events emitted after the deleted characters. Trace the text-selection change event that causes the prompt character to be announced; done means deleted characters are read, but subsequent backspace presses produce no spurious selection event.

Written by the indexing model from the issue text.