Fix TUI status visibility around streamed output (#33105)

## What changed

- Hide the working-status indicator when finalized assistant output is committed to the transcript, including single-line final answers.
- Restore the indicator when image generation begins during a running task after its streamed preamble has been flushed.

## Testing

- Add snapshot coverage for a single-line final answer and for image generation following a single-line preamble.

GitOrigin-RevId: e294a11cda3f0ce46f2bbed1be2a1380c9c73085

codex-rs/tui/src/chatwidget/tests/exec_flow.rs

@@ -877,6 +877,30 @@ async fn view_image_tool_call_preserves_foreign_path() {
     assert_chatwidget_snapshot!("foreign_image_attachment_history_snapshot", combined);
 }
 
+#[tokio::test]
+async fn image_generation_begin_restores_working_status_after_single_line_preamble() {
+    let (mut chat, _rx, _op_rx) = make_chatwidget_manual(/*model_override*/ None).await;
+
+    chat.on_task_started();
+    chat.on_agent_message_delta("Generating an image.".to_string());
+    chat.on_image_generation_begin();
+
+    assert!(chat.bottom_pane.is_task_running());
+    assert!(chat.bottom_pane.status_indicator_visible());
+
+    let width: u16 = 80;
+    let height = chat.desired_height(width);
+    let mut terminal = ratatui::Terminal::new(ratatui::backend::TestBackend::new(width, height))
+        .expect("create terminal");
+    terminal
+        .draw(|frame| chat.render(frame.area(), frame.buffer_mut()))
+        .expect("draw image generation status");
+    assert_chatwidget_snapshot!(
+        "image_generation_begin_restores_working_status",
+        normalized_backend_snapshot(terminal.backend())
+    );
+}
+
 #[tokio::test]
 async fn image_generation_call_adds_history_cell() {
     let (mut chat, mut rx, _op_rx) = make_chatwidget_manual(/*model_override*/ None).await;