AmazonQ - @smithy/util-base64: toBase64 encoder function only accepts string | Uint8Array.

## Problem
The following message is obtained when sending a query to Amazon Q, in a chat with existing history:

@smithy/util-base64: toBase64 encoder function only accepts string | Uint8Array.

The issue does not occur in a new chat.

## Steps to reproduce the issue

1. Go to 'Amazon Q chat panel in VScode'
2. Select any model.
3. Send a query in a chat with existing history.
4. See error "@smithy/util-base64: toBase64 encoder function only accepts string | Uint8Array."

## Expected behavior
Get a response from LLM.

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

- OS: Windows 11
- Visual Studio Code version: 1.106.3
- AWS Toolkit version: n/a
- Amazon Q version: 1.106.0

Start in the Amazon Q chat panel in VS Code and reproduce the error by sending a query in a chat with existing history, using the system details in the report. Trace how existing chat history reaches the Amazon Q request and identify where the reported base64 input is produced. Done means the same query returns an LLM response without the encoder error.

Written by the indexing model from the issue text.