Re-add markdown streaming (#2029)

Wait for newlines, then render markdown on a line by line basis. Word wrap it for the current terminal size and then spit it out line by line into the UI. Also adds tests and fixes some UI regressions.

codex-rs/tui/src/app_event.rs

@@ -7,6 +7,7 @@ use crate::app::ChatWidgetArgs;
 use crate::slash_command::SlashCommand;
 
 #[allow(clippy::large_enum_variant)]
+#[derive(Debug)]
 pub(crate) enum AppEvent {
     CodexEvent(Event),
 
@@ -50,6 +51,10 @@ pub(crate) enum AppEvent {
 
     InsertHistory(Vec<Line<'static>>),
 
+    StartCommitAnimation,
+    StopCommitAnimation,
+    CommitTick,
+
     /// Onboarding: result of login_with_chatgpt.
     OnboardingAuthComplete(Result<(), String>),
     OnboardingComplete(ChatWidgetArgs),