Move TUI on top of app server (parallel code) (#14717)

This PR replicates the `tui` code directory and creates a temporary
parallel `tui_app_server` directory. It also implements a new feature
flag `tui_app_server` to select between the two tui implementations.

Once the new app-server-based TUI is stabilized, we'll delete the old
`tui` directory and feature flag.

codex-rs/tui_app_server/tests/suite/status_indicator.rs

@@ -0,0 +1,24 @@
+//! Regression test: ensure that `StatusIndicatorWidget` sanitises ANSI escape
+//! sequences so that no raw `\x1b` bytes are written into the backing
+//! buffer.  Rendering logic is tricky to unit‑test end‑to‑end, therefore we
+//! verify the *public* contract of `ansi_escape_line()` which the widget now
+//! relies on.
+
+use codex_ansi_escape::ansi_escape_line;
+
+#[test]
+fn ansi_escape_line_strips_escape_sequences() {
+    let text_in_ansi_red = "\x1b[31mRED\x1b[0m";
+
+    // The returned line must contain three printable glyphs and **no** raw
+    // escape bytes.
+    let line = ansi_escape_line(text_in_ansi_red);
+
+    let combined: String = line
+        .spans
+        .iter()
+        .map(|span| span.content.to_string())
+        .collect();
+
+    assert_eq!(combined, "RED");
+}