Follow up to https://github.com/zed-industries/zed/pull/53349 as I was
still seeing cases where content would overlap with buttons.

<img width="500" height="266" alt="Screenshot 2026-04-08 at 6  53@2x"
src="https://github.com/user-attachments/assets/e58742d4-a736-4509-9a29-35c135ddb980"
/>

I believe this PR makes the whole message editor more solid against
these cases because we're relying on `flex_basis` now to set the
max-width, which runs before we do any editor mode calculations. Tested
this a bunch and it seems to be pretty stable.

Release Notes:

- N/A