Long code blocks in chat can't be scrolled horizontally/vertically; no way to delete a previously sent message

1. When a code block or terminal output box in the chat panel contains content wider or taller than the visible area, there's no scrollbar/way to scroll right or to the bottom to see the rest of the content.
2. There's no way to delete/remove a message the user already sent in an ongoing conversation (e.g. to clean up an accidental or unnecessary message) — only stopping generation or starting a new conversation is possible.

**Environment:** VSCode 1.136.0, Claude Code extension 2.1.260, Linux

Reproduce both behaviors in the chat panel using VSCode 1.136.0, Claude Code extension 2.1.260, and Linux: a code block or terminal output larger than the visible area, then an already-sent message. Trace the chat panel entry points and existing message controls; done means long content can be scrolled in both directions and a user can remove a previously sent message.

Written by the indexing model from the issue text.