Follow-up to https://github.com/zed-industries/zed/pull/52608

This PR adds a new iteration to the thinking block display design after
some internal round of feedback. It turns out, we had some people
appreciating the auto-collapse when thinking is done; thinking content
isn't too useful afterwards and it is just more content _to to he
model_, not the user. I also liked the one old but it definitely has the
issue of being a jarring layout shift when it wraps up. So that's why
I'm keeping what I introduced in the PR linked above as a setting, so
that anyone who feels strongly about the default (auto-expand, and
auto-collapse) can change that.

Release Notes:

- N/A