Follow up to https://github.com/zed-industries/zed/pull/61245. That PR
introduced a bug where the thread controls would appear at seemingly
random tool boundaries as opposed to only at the end of each turn. It
caused this sort of behavior, where we'd have many instances of those
buttons showing up:

<img width="500" alt="Screenshot 2026-07-20 at 4  03 2@2x"
src="https://github.com/user-attachments/assets/6e6f716e-adb5-4532-9f89-e955398cda23"
/>

Alongside with fixing it, this PR is also making the "scroll to top"
only show up only at the very last turn. I think that makes more sense
given you'd be most likely interested to go to the top when you're at
the bottom, rather than at every turn, but I could be persuaded
otherwise; not holding that too strongly.

Release Notes:

- N/A