PgUp/PgDown moves chat window viewport

## Problem

## Steps to reproduce the issue

1. Open Amazon Q tab.
2. Start chat. (It might help to have a longer chat history, so ask for e.g. Lorem Ipsum or use an existing long chat).
3. While the text input area is active, press PgUp or PgDown.
4. Chat window viewport will now misalign.

## Expected behavior

Viewport does not move.
Text cursor moves up and down inside text box if possible -- "Natural text editing" behaviour.

## System details (run `AWS: About` and/or `Amazon Q: About`)

- OS: Windows 11 Enterprise / 10.0.22631 Build 22631
- Visual Studio Code version: vscode 1.107.1 (choco install) / 994fd12f8d3a5aa16f17d42c041e5809167e845a / x64
- AWS Toolkit version: Not separately installed (integrated into Amazon Q)
- Amazon Q version: amazonwebservices.amazon-q-vscode@1.109.0

Start in the Amazon Q tab with a long chat and an active text input, then reproduce the PgUp/PgDown behavior on the reported Windows and VS Code setup. Trace the chat viewport and text-input handling from that entry point. Done means the viewport stays aligned while the text cursor moves naturally within the input when possible.

Written by the indexing model from the issue text.