agent: Add message editor UI improvements (#29594)

Probably the most relevant change in this PR is the commented out (still
pending) line number diffs. Aside from this, commits are pretty
descriptive.

Release Notes:

- N/A
7e25460708Danilo Leal committed on 4/29/2025, 12:38:44 PM· committed by GitHubparent9b37206
2 files changedLine totals unavailable