Self-Review Checklist:

- [x] I've reviewed my own diff for quality, security, and reliability
- [x] Unsafe blocks (if any) have justifying comments
- [x] The content is consistent with the [UI/UX
checklist](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
- [x] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

Closes #52132.

*This PR previously also fixes the "expanded message editor not taking
up full height" part, but #52545 has already fixed that. Yet it seems to
leave a new issue (that was previously not revealed) behind, as
follows.*

This PR fixes the `Full + ExcludeOverscrollMargin` editor mode (which
the agent panel message editor uses in expanded mode), which could
jitter while typing because render-time layout and scroll-position
updates were clamping against different effective
`scroll_beyond_last_line` policies. This PR fixes that inconsistency so
the expanded editor stays stable while typing, and adds a regression
test covering `ExcludeOverscrollMargin` scroll clamping.


https://github.com/user-attachments/assets/86abf04d-c1a1-419b-96d0-8ca097c0acb0


https://github.com/user-attachments/assets/03dbdc3c-f58e-4378-8c6a-4bda1ae425c8

Release Notes:

- Fixed the expanded Agent Panel message editor so it no longer jitters
while typing.

---------

Co-authored-by: MrSubidubi <finn@zed.dev>