With these newer models that come with different thinking levels, it's
become more frequent to want to see what the thinking is outputting.
Thus far in Zed, the thinking block would show up automatically
collapsed and every time you wanted to see it, you had to expand it
manually. This PR changes that by making the thinking block
automatically _expanded_ instead, but as soon as it's done, it collapses
again.

Release Notes:

- Agent: Improved visibility of thinking blocks by making them
auto-expanded while in progress.