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/chatwidget/agent.rs

@@ -0,0 +1,53 @@
+use std::sync::Arc;
+
+use codex_core::codex_wrapper::CodexConversation;
+use codex_core::codex_wrapper::init_codex;
+use codex_core::config::Config;
+use codex_core::protocol::Op;
+use tokio::sync::mpsc::UnboundedSender;
+use tokio::sync::mpsc::unbounded_channel;
+
+use crate::app_event::AppEvent;
+use crate::app_event_sender::AppEventSender;
+
+/// Spawn the agent bootstrapper and op forwarding loop, returning the
+/// `UnboundedSender<Op>` used by the UI to submit operations.
+pub(crate) fn spawn_agent(config: Config, app_event_tx: AppEventSender) -> UnboundedSender<Op> {
+    let (codex_op_tx, mut codex_op_rx) = unbounded_channel::<Op>();
+
+    let app_event_tx_clone = app_event_tx.clone();
+    tokio::spawn(async move {
+        let CodexConversation {
+            codex,
+            session_configured,
+            ..
+        } = match init_codex(config).await {
+            Ok(vals) => vals,
+            Err(e) => {
+                // TODO: surface this error to the user.
+                tracing::error!("failed to initialize codex: {e}");
+                return;
+            }
+        };
+
+        // Forward the captured `SessionInitialized` event that was consumed
+        // inside `init_codex()` so it can be rendered in the UI.
+        app_event_tx_clone.send(AppEvent::CodexEvent(session_configured.clone()));
+        let codex = Arc::new(codex);
+        let codex_clone = codex.clone();
+        tokio::spawn(async move {
+            while let Some(op) = codex_op_rx.recv().await {
+                let id = codex_clone.submit(op).await;
+                if let Err(e) = id {
+                    tracing::error!("failed to submit op: {e}");
+                }
+            }
+        });
+
+        while let Ok(event) = codex.next_event().await {
+            app_event_tx_clone.send(AppEvent::CodexEvent(event));
+        }
+    });
+
+    codex_op_tx
+}