After pasting text to chat, undo does not work correctly

## Problem

When typing to Chat and pasting some text, undo does not work as intended. Undo removed last typed character and leaves pasted content as-is.

## Steps to reproduce the issue

https://github.com/user-attachments/assets/4b92a38b-f1f2-40f9-a5e7-b5a64833d64e

1. Open Amazon Q developer chat
2. Type something
3. Paste some text (CTRL + V)
4. Undo (CTRL + Z)
5. Last TYPED character is remove and pasted content stays

## Expected behavior

Pasted content is removed.

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

- OS: Microsoft Windows 11 Pro
- Version: 10.0.26100 Build 26100
- WSL OS: Linux x64 6.6.87.2-microsoft-standard-WSL2
- WSL OS version: Amazon Linux 2023
- Visual Studio Code version: 1.105.0
- Visual Studio Code extension host: 1.105.0
- Amazon Q version: 1.99.0
- Node: 22.19.0

Reproduce the issue in Amazon Q developer chat on Windows using the listed steps, then trace the chat input's paste and undo handling in the VS Code extension. Done means Ctrl+Z removes the pasted content rather than the last typed character, with behavior verified against the reproduction steps.

Written by the indexing model from the issue text.