agent: Fix long previous user message double scroll (#33056)

Previously, if editing a long previous user message in the thread, you'd
have a double scroll situation because the editor used in that case had
its max number of lines capped. To solve that, I made the `max_lines` in
the editor `AutoHeight` mode optional, allowing me to not pass any
arbitrary number to the previous user message editor, and ultimately,
solving the double scroll problem by not having any scroll at all.

Release Notes:

- agent: Fixed double scroll that happened when editing a long previous
user message.

@ConradIrwin adding you as a reviewer as I'm touching editor code
here... want to be careful. :)

crates/repl/src/notebook/cell.rs

@@ -179,7 +179,7 @@ impl Cell {
                     let mut editor = Editor::new(
                         EditorMode::AutoHeight {
                             min_lines: 1,
-                            max_lines: 1024,
+                            max_lines: Some(1024),
                         },
                         multi_buffer,
                         None,