agent: Cleanup `message_editor` (#28614)

This PR splits up the rendering of the message editor into multiple
functions. Previously we had a single `h_flex()...` expression which
spanned across 550 lines, `cargo fmt` stopped working.

Release Notes:

- N/A
429d4580cfBennet Bo Fenner committed on 4/12/2025, 12:06:49 AM· committed by GitHubparent62ebae9
1 file changedLine totals unavailable