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/all.rs

@@ -0,0 +1,9 @@
+// Single integration test binary that aggregates all test modules.
+// The submodules live in `tests/suite/`.
+#[cfg(feature = "vt100-tests")]
+mod test_backend;
+
+#[allow(unused_imports)]
+use codex_cli as _; // Keep dev-dep for cargo-shear; tests spawn the codex binary.
+
+mod suite;