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/src/app_server_tui_dispatch.rs

@@ -0,0 +1,45 @@
+use std::future::Future;
+
+use crate::Cli;
+use codex_core::config::Config;
+use codex_core::config::ConfigOverrides;
+use codex_core::features::Feature;
+
+pub(crate) fn app_server_tui_config_inputs(
+    cli: &Cli,
+) -> std::io::Result<(Vec<(String, toml::Value)>, ConfigOverrides)> {
+    let mut raw_overrides = cli.config_overrides.raw_overrides.clone();
+    if cli.web_search {
+        raw_overrides.push("web_search=\"live\"".to_string());
+    }
+
+    let cli_kv_overrides = codex_utils_cli::CliConfigOverrides { raw_overrides }
+        .parse_overrides()
+        .map_err(|err| std::io::Error::new(std::io::ErrorKind::InvalidInput, err))?;
+
+    let config_overrides = ConfigOverrides {
+        cwd: cli.cwd.clone(),
+        config_profile: cli.config_profile.clone(),
+        ..Default::default()
+    };
+
+    Ok((cli_kv_overrides, config_overrides))
+}
+
+pub(crate) async fn should_use_app_server_tui_with<F, Fut>(
+    cli: &Cli,
+    load_config: F,
+) -> std::io::Result<bool>
+where
+    F: FnOnce(Vec<(String, toml::Value)>, ConfigOverrides) -> Fut,
+    Fut: Future<Output = std::io::Result<Config>>,
+{
+    let (cli_kv_overrides, config_overrides) = app_server_tui_config_inputs(cli)?;
+    let config = load_config(cli_kv_overrides, config_overrides).await?;
+
+    Ok(config.features.enabled(Feature::TuiAppServer))
+}
+
+pub async fn should_use_app_server_tui(cli: &Cli) -> std::io::Result<bool> {
+    should_use_app_server_tui_with(cli, Config::load_with_cli_overrides_and_harness_overrides).await
+}