'@' add content disappeared from the chat window

## Problem

## Steps to reproduce the issue
I’m not able to see the @ button that used to appear near the Enter key just 8 hours ago. Now, I’m also not receiving any responses in the Pro version chat.

## Expected behavior

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

- OS: W 11
- Visual Studio Code version: Latest
- AWS Toolkit version: Auto update: on
- Amazon Q version: Auto update: on

Start by reproducing the missing @ button and absent Pro chat responses in the Amazon Q chat window using the reported Windows 11 and latest VS Code, AWS Toolkit, and Amazon Q versions. Done means the @ control is visible near Enter and the Pro chat responds again; no source files or tests are identified in the issue.

Written by the indexing model from the issue text.