Follow-up to zed-industries/zed#60721.

The Taffy 0.12 upgrade exposed that the message height limit was applied
to its grid parent while percentage-sized children retained the Markdown
content's intrinsic height. Long messages therefore painted over the
changed-files summary instead of scrolling.

Move the height limit to the scroll container itself, matching the
commit view. Add a GPUI regression test that verifies the viewport stays
bounded, remains scrollable, and does not overlap the changed-files
list.

> On `main`:
<img width="697" height="873" alt="Screenshot 2026-07-17 at 14 59 21"
src="https://github.com/user-attachments/assets/51e6cc0e-aeac-4ad1-a6af-2120efcef901"
/>

---

Release Notes:

- N/A