Follow up to https://github.com/zed-industries/zed/pull/51525
Closes https://github.com/zed-industries/zed/issues/52452

This PR further refines the behavior of thinking blocks in the agent
panel. In the PR linked above, I had previously made it auto-expand
while running but then auto-collapse when finished. Although that
reduced the sense of staleness when the model is thinking for too long,
it caused layout shift that many found jarring.

Therefore, this PR changes the behavior so that, by default, thinking
blocks render "almost" fully expanded. They will have a max-height and
will auto-scroll as content streams in. Therefore, this design fully
removes layout shift and still allows you to sort of follow along the
generated thinking content, even though sometimes it can be fast, in
which case you can always fully expand it. Lastly, I'm also adding a
"thinking display" setting that allows to choose between the "automatic"
behavior (what I just described, the default), always expanded, or
always collapsed.

Here's a preview:


https://github.com/user-attachments/assets/c96c89c7-40ed-4e9b-9ffc-f70b0659be47

Release Notes:

- Agent: Refined thinking block display, removing layout shift while
still allowing it to be readable while it streams in. It comes together
with a "Thinking Display" setting to control the behavior