This fixes an issue where long placeholders would be cut off, e.g. in a
Claude Code thread:

<img width="387" height="115" alt="image"
src="https://github.com/user-attachments/assets/831a54aa-cf2b-4d87-af86-e368a5936f6b"
/>

Now:

<img width="354" height="115" alt="image"
src="https://github.com/user-attachments/assets/e5df5e05-0869-4db2-8dee-38611263191c"
/>


Most of the changes in this PR are caused by us requiring `&mut Window`
in `set_placeholder_text`.

Release Notes:

- Fixed an issue where placeholders inside editors would not wrap

---------

Co-authored-by: Agus Zubiaga <agus@zed.dev>