agent: Apply soft-wrap when message editor is expanded (#28716)

Release Notes:

- N/A

crates/agent/src/message_editor.rs

@@ -93,6 +93,7 @@ impl MessageEditor {
             );
             editor.set_placeholder_text("Ask anything, @ to mention, ↑ to select", cx);
             editor.set_show_indent_guides(false, cx);
+            editor.set_soft_wrap();
             editor.set_context_menu_options(ContextMenuOptions {
                 min_entries_visible: 12,
                 max_entries_visible: 12,