Remove the legacy TUI split (#15922)

This is the part 1 of 2 PRs that will delete the `tui` /
`tui_app_server` split. This part simply deletes the existing `tui`
directory and marks the `tui_app_server` feature flag as removed. I left
the `tui_app_server` feature flag in place for now so its presence
doesn't result in an error. It is simply ignored.

Part 2 will rename the `tui_app_server` directory `tui`. I did this as
two parts to reduce visible code churn.

codex-rs/tui/src/notifications/mod.rs

@@ -1,156 +0,0 @@
-mod bel;
-mod osc9;
-
-use std::env;
-use std::io;
-
-use bel::BelBackend;
-use codex_core::config::types::NotificationMethod;
-use osc9::Osc9Backend;
-
-#[derive(Debug)]
-pub enum DesktopNotificationBackend {
-    Osc9(Osc9Backend),
-    Bel(BelBackend),
-}
-
-impl DesktopNotificationBackend {
-    pub fn for_method(method: NotificationMethod) -> Self {
-        match method {
-            NotificationMethod::Auto => {
-                if supports_osc9() {
-                    Self::Osc9(Osc9Backend)
-                } else {
-                    Self::Bel(BelBackend)
-                }
-            }
-            NotificationMethod::Osc9 => Self::Osc9(Osc9Backend),
-            NotificationMethod::Bel => Self::Bel(BelBackend),
-        }
-    }
-
-    pub fn method(&self) -> NotificationMethod {
-        match self {
-            DesktopNotificationBackend::Osc9(_) => NotificationMethod::Osc9,
-            DesktopNotificationBackend::Bel(_) => NotificationMethod::Bel,
-        }
-    }
-
-    pub fn notify(&mut self, message: &str) -> io::Result<()> {
-        match self {
-            DesktopNotificationBackend::Osc9(backend) => backend.notify(message),
-            DesktopNotificationBackend::Bel(backend) => backend.notify(message),
-        }
-    }
-}
-
-pub fn detect_backend(method: NotificationMethod) -> DesktopNotificationBackend {
-    DesktopNotificationBackend::for_method(method)
-}
-
-fn supports_osc9() -> bool {
-    if env::var_os("WT_SESSION").is_some() {
-        return false;
-    }
-    // Prefer TERM_PROGRAM when present, but keep fallbacks for shells/launchers
-    // that don't set it (e.g., tmux/ssh) to avoid regressing OSC 9 support.
-    if matches!(
-        env::var("TERM_PROGRAM").ok().as_deref(),
-        Some("WezTerm" | "ghostty")
-    ) {
-        return true;
-    }
-    // iTerm still provides a strong session signal even when TERM_PROGRAM is missing.
-    if env::var_os("ITERM_SESSION_ID").is_some() {
-        return true;
-    }
-    // TERM-based hints cover kitty/wezterm setups without TERM_PROGRAM.
-    matches!(
-        env::var("TERM").ok().as_deref(),
-        Some("xterm-kitty" | "wezterm" | "wezterm-mux")
-    )
-}
-
-#[cfg(test)]
-mod tests {
-    use super::detect_backend;
-    use codex_core::config::types::NotificationMethod;
-    use serial_test::serial;
-    use std::ffi::OsString;
-
-    struct EnvVarGuard {
-        key: &'static str,
-        original: Option<OsString>,
-    }
-
-    impl EnvVarGuard {
-        fn set(key: &'static str, value: &str) -> Self {
-            let original = std::env::var_os(key);
-            unsafe {
-                std::env::set_var(key, value);
-            }
-            Self { key, original }
-        }
-
-        fn remove(key: &'static str) -> Self {
-            let original = std::env::var_os(key);
-            unsafe {
-                std::env::remove_var(key);
-            }
-            Self { key, original }
-        }
-    }
-
-    impl Drop for EnvVarGuard {
-        fn drop(&mut self) {
-            unsafe {
-                match &self.original {
-                    Some(value) => std::env::set_var(self.key, value),
-                    None => std::env::remove_var(self.key),
-                }
-            }
-        }
-    }
-
-    #[test]
-    fn selects_osc9_method() {
-        assert!(matches!(
-            detect_backend(NotificationMethod::Osc9),
-            super::DesktopNotificationBackend::Osc9(_)
-        ));
-    }
-
-    #[test]
-    fn selects_bel_method() {
-        assert!(matches!(
-            detect_backend(NotificationMethod::Bel),
-            super::DesktopNotificationBackend::Bel(_)
-        ));
-    }
-
-    #[test]
-    #[serial]
-    fn auto_prefers_bel_without_hints() {
-        let _term = EnvVarGuard::remove("TERM");
-        let _term_program = EnvVarGuard::remove("TERM_PROGRAM");
-        let _iterm = EnvVarGuard::remove("ITERM_SESSION_ID");
-        let _wt = EnvVarGuard::remove("WT_SESSION");
-        assert!(matches!(
-            detect_backend(NotificationMethod::Auto),
-            super::DesktopNotificationBackend::Bel(_)
-        ));
-    }
-
-    #[test]
-    #[serial]
-    fn auto_uses_osc9_for_iterm() {
-        let _term = EnvVarGuard::remove("TERM");
-        let _term_program = EnvVarGuard::remove("TERM_PROGRAM");
-        let _iterm = EnvVarGuard::set("ITERM_SESSION_ID", "abc");
-        let _wt = EnvVarGuard::remove("WT_SESSION");
-        assert!(matches!(
-            detect_backend(NotificationMethod::Auto),
-            super::DesktopNotificationBackend::Osc9(_)
-        ));
-    }
-}