AI Assistant: autoscroll won't stop on block follow-up completions

The autoscroll is not working well when the request being completed is a follow-up to a previous one.

Steps to reproduce:

- add an AI Assistant block and ask for a long completion (longer than the height available, for example: "10 paragraphs about the history of France")
- when the completion is done, perform a follow-up request; for example, change the tone to "Passionate"

Expected behavior:

- the autoscroll keep scrolling to show the last parts of the completion
- any scroll event from the user disables the autoscroll

Observed behavior:

- the last 2 or 3 lines of the completion are hidden behind the AI Assistant input bar
- user scrolls don't stop the autoscroll
- video:

https://github.com/Automattic/jetpack/assets/6760046/ff62a937-41c1-4554-869e-f73ec8913d73

Reproduce the issue in the AI Assistant block using a long completion followed by a tone-changing follow-up. Start by tracing the block's completion autoscroll and user-scroll handling; done means the final lines remain visible and any user scroll disables autoscroll during follow-up completions.

Written by the indexing model from the issue text.