update composer + user message styling (#4240)

Changes:

- the composer and user messages now have a colored background that
stretches the entire width of the terminal.
- the prompt character was changed from a cyan `▌` to a bold `›`.
- the "working" shimmer now follows the "dark gray" color of the
terminal, better matching the terminal's color scheme

| Terminal + Background        | Screenshot |
|------------------------------|------------|
| iTerm with dark bg | <img width="810" height="641" alt="Screenshot
2025-09-25 at 11 44 52 AM"
src="https://github.com/user-attachments/assets/1317e579-64a9-4785-93e6-98b0258f5d92"
/> |
| iTerm with light bg | <img width="845" height="540" alt="Screenshot
2025-09-25 at 11 46 29 AM"
src="https://github.com/user-attachments/assets/e671d490-c747-4460-af0b-3f8d7f7a6b8e"
/> |
| iTerm with color bg | <img width="825" height="564" alt="Screenshot
2025-09-25 at 11 47 12 AM"
src="https://github.com/user-attachments/assets/141cda1b-1164-41d5-87da-3be11e6a3063"
/> |
| Terminal.app with dark bg | <img width="577" height="367"
alt="Screenshot 2025-09-25 at 11 45 22 AM"
src="https://github.com/user-attachments/assets/93fc4781-99f7-4ee7-9c8e-3db3cd854fe5"
/> |
| Terminal.app with light bg | <img width="577" height="367"
alt="Screenshot 2025-09-25 at 11 46 04 AM"
src="https://github.com/user-attachments/assets/19bf6a3c-91e0-447b-9667-b8033f512219"
/> |
| Terminal.app with color bg | <img width="577" height="367"
alt="Screenshot 2025-09-25 at 11 45 50 AM"
src="https://github.com/user-attachments/assets/dd7c4b5b-342e-4028-8140-f4e65752bd0b"
/> |

codex-rs/tui/src/test_backend.rs

@@ -0,0 +1,123 @@
+use std::fmt::{self};
+use std::io::Write;
+use std::io::{self};
+
+use ratatui::prelude::CrosstermBackend;
+
+use ratatui::backend::Backend;
+use ratatui::backend::ClearType;
+use ratatui::backend::WindowSize;
+use ratatui::buffer::Cell;
+use ratatui::layout::Position;
+use ratatui::layout::Size;
+
+/// This wraps a CrosstermBackend and a vt100::Parser to mock
+/// a "real" terminal.
+///
+/// Importantly, this wrapper avoids calling any crossterm methods
+/// which write to stdout regardless of the writer. This includes:
+/// - getting the terminal size
+/// - getting the cursor position
+pub struct VT100Backend {
+    crossterm_backend: CrosstermBackend<vt100::Parser>,
+}
+
+impl VT100Backend {
+    /// Creates a new `TestBackend` with the specified width and height.
+    pub fn new(width: u16, height: u16) -> Self {
+        Self {
+            crossterm_backend: CrosstermBackend::new(vt100::Parser::new(height, width, 0)),
+        }
+    }
+
+    pub fn vt100(&self) -> &vt100::Parser {
+        self.crossterm_backend.writer()
+    }
+}
+
+impl Write for VT100Backend {
+    fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
+        self.crossterm_backend.writer_mut().write(buf)
+    }
+
+    fn flush(&mut self) -> io::Result<()> {
+        self.crossterm_backend.writer_mut().flush()
+    }
+}
+
+impl fmt::Display for VT100Backend {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "{}", self.crossterm_backend.writer().screen().contents())
+    }
+}
+
+impl Backend for VT100Backend {
+    fn draw<'a, I>(&mut self, content: I) -> io::Result<()>
+    where
+        I: Iterator<Item = (u16, u16, &'a Cell)>,
+    {
+        self.crossterm_backend.draw(content)?;
+        Ok(())
+    }
+
+    fn hide_cursor(&mut self) -> io::Result<()> {
+        self.crossterm_backend.hide_cursor()?;
+        Ok(())
+    }
+
+    fn show_cursor(&mut self) -> io::Result<()> {
+        self.crossterm_backend.show_cursor()?;
+        Ok(())
+    }
+
+    fn get_cursor_position(&mut self) -> io::Result<Position> {
+        Ok(self.vt100().screen().cursor_position().into())
+    }
+
+    fn set_cursor_position<P: Into<Position>>(&mut self, position: P) -> io::Result<()> {
+        self.crossterm_backend.set_cursor_position(position)
+    }
+
+    fn clear(&mut self) -> io::Result<()> {
+        self.crossterm_backend.clear()
+    }
+
+    fn clear_region(&mut self, clear_type: ClearType) -> io::Result<()> {
+        self.crossterm_backend.clear_region(clear_type)
+    }
+
+    fn append_lines(&mut self, line_count: u16) -> io::Result<()> {
+        self.crossterm_backend.append_lines(line_count)
+    }
+
+    fn size(&self) -> io::Result<Size> {
+        Ok(self.vt100().screen().size().into())
+    }
+
+    fn window_size(&mut self) -> io::Result<WindowSize> {
+        Ok(WindowSize {
+            columns_rows: self.vt100().screen().size().into(),
+            // Arbitrary size, we don't rely on this in testing.
+            pixels: Size {
+                width: 640,
+                height: 480,
+            },
+        })
+    }
+
+    fn flush(&mut self) -> io::Result<()> {
+        self.crossterm_backend.writer_mut().flush()
+    }
+
+    fn scroll_region_up(&mut self, region: std::ops::Range<u16>, scroll_by: u16) -> io::Result<()> {
+        self.crossterm_backend.scroll_region_up(region, scroll_by)
+    }
+
+    fn scroll_region_down(
+        &mut self,
+        region: std::ops::Range<u16>,
+        scroll_by: u16,
+    ) -> io::Result<()> {
+        self.crossterm_backend.scroll_region_down(region, scroll_by)
+    }
+}