refactor transcript view to handle HistoryCells (#3538)

No (intended) functional change.

This refactors the transcript view to hold a list of HistoryCells
instead of a list of Lines. This simplifies and makes much of the logic
more robust, as well as laying the groundwork for future changes, e.g.
live-updating history cells in the transcript.

Similar to #2879 in goal. Fixes #2755.

codex-rs/tui/src/history_cell.rs

@@ -43,6 +43,7 @@ use ratatui::style::Stylize;
 use ratatui::widgets::Paragraph;
 use ratatui::widgets::WidgetRef;
 use ratatui::widgets::Wrap;
+use std::any::Any;
 use std::collections::HashMap;
 use std::io::Cursor;
 use std::path::Path;
@@ -69,7 +70,7 @@ pub(crate) enum PatchEventType {
 /// Represents an event to display in the conversation history. Returns its
 /// `Vec<Line<'static>>` representation to make it easier to display in a
 /// scrollable list.
-pub(crate) trait HistoryCell: std::fmt::Debug + Send + Sync {
+pub(crate) trait HistoryCell: std::fmt::Debug + Send + Sync + Any {
     fn display_lines(&self, width: u16) -> Vec<Line<'static>>;
 
     fn transcript_lines(&self) -> Vec<Line<'static>> {
@@ -89,9 +90,15 @@ pub(crate) trait HistoryCell: std::fmt::Debug + Send + Sync {
     }
 }
 
+impl dyn HistoryCell {
+    pub(crate) fn as_any(&self) -> &dyn Any {
+        self
+    }
+}
+
 #[derive(Debug)]
 pub(crate) struct UserHistoryCell {
-    message: String,
+    pub message: String,
 }
 
 impl HistoryCell for UserHistoryCell {