Follow up to https://github.com/zed-industries/zed/pull/32765.
This PR creates a slot for the `message_editor::create_editor` to allow
using different values for min and max lines. In practice, the panel's
main editor now has a minimum of 4 lines, whereas the previous message
editor has just one. This makes the layout shift when clicking on a
previous message to edit it much smaller.
Release Notes:
- agent: Improved layout shift when clicking to edit a previous sent
message.