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/tests/suite/vt100_live_commit.rs

@@ -1,12 +1,12 @@
 #![cfg(feature = "vt100-tests")]
 
-use ratatui::backend::TestBackend;
+use crate::test_backend::VT100Backend;
 use ratatui::layout::Rect;
 use ratatui::text::Line;
 
 #[test]
 fn live_001_commit_on_overflow() {
-    let backend = TestBackend::new(20, 6);
+    let backend = VT100Backend::new(20, 6);
     let mut term = match codex_tui::custom_terminal::Terminal::with_options(backend) {
         Ok(t) => t,
         Err(e) => panic!("failed to construct terminal: {e}"),
@@ -26,27 +26,12 @@ fn live_001_commit_on_overflow() {
     let commit_rows = rb.drain_commit_ready(3);
     let lines: Vec<Line<'static>> = commit_rows.into_iter().map(|r| r.text.into()).collect();
 
-    let mut buf: Vec<u8> = Vec::new();
-    codex_tui::insert_history::insert_history_lines_to_writer(&mut term, &mut buf, lines);
+    codex_tui::insert_history::insert_history_lines(&mut term, lines);
 
-    let mut parser = vt100::Parser::new(6, 20, 0);
-    parser.process(&buf);
-    let screen = parser.screen();
+    let screen = term.backend().vt100().screen();
 
     // The words "one" and "two" should appear above the viewport.
-    let mut joined = String::new();
-    for row in 0..6 {
-        for col in 0..20 {
-            if let Some(cell) = screen.cell(row, col) {
-                if let Some(ch) = cell.contents().chars().next() {
-                    joined.push(ch);
-                } else {
-                    joined.push(' ');
-                }
-            }
-        }
-        joined.push('\n');
-    }
+    let joined = screen.contents();
     assert!(
         joined.contains("one"),
         "expected committed 'one' to be visible\n{joined}"
@@ -57,39 +42,3 @@ fn live_001_commit_on_overflow() {
     );
     // The last three (three,four,five) remain in the live ring, not committed here.
 }
-
-#[test]
-fn live_002_pre_scroll_and_commit() {
-    let backend = TestBackend::new(20, 6);
-    let mut term = match codex_tui::custom_terminal::Terminal::with_options(backend) {
-        Ok(t) => t,
-        Err(e) => panic!("failed to construct terminal: {e}"),
-    };
-    // Viewport not at bottom: y=3
-    let area = Rect::new(0, 3, 20, 1);
-    term.set_viewport_area(area);
-
-    let mut rb = codex_tui::live_wrap::RowBuilder::new(20);
-    rb.push_fragment("alpha\n");
-    rb.push_fragment("beta\n");
-    rb.push_fragment("gamma\n");
-    rb.push_fragment("delta\n");
-
-    // Keep 3, commit 1.
-    let commit_rows = rb.drain_commit_ready(3);
-    let lines: Vec<Line<'static>> = commit_rows.into_iter().map(|r| r.text.into()).collect();
-
-    let mut buf: Vec<u8> = Vec::new();
-    codex_tui::insert_history::insert_history_lines_to_writer(&mut term, &mut buf, lines);
-    let s = String::from_utf8_lossy(&buf);
-
-    // Expect a SetScrollRegion to [area.top()+1 .. screen_height] and a cursor move to top of that region.
-    assert!(
-        s.contains("\u{1b}[4;6r"),
-        "expected pre-scroll region 4..6, got: {s:?}"
-    );
-    assert!(
-        s.contains("\u{1b}[4;1H"),
-        "expected cursor CUP 4;1H, got: {s:?}"
-    );
-}