fix error on missing notifications in [tui] (#3867)

Fixes #3811.

codex-rs/core/src/config.rs

@@ -1163,6 +1163,7 @@ pub fn log_dir(cfg: &Config) -> std::io::Result<PathBuf> {
 #[cfg(test)]
 mod tests {
     use crate::config_types::HistoryPersistence;
+    use crate::config_types::Notifications;
 
     use super::*;
     use pretty_assertions::assert_eq;
@@ -1201,6 +1202,19 @@ persistence = "none"
         );
     }
 
+    #[test]
+    fn tui_config_missing_notifications_field_defaults_to_disabled() {
+        let cfg = r#"
+[tui]
+"#;
+
+        let parsed = toml::from_str::<ConfigToml>(cfg)
+            .expect("TUI config without notifications should succeed");
+        let tui = parsed.tui.expect("config should include tui section");
+
+        assert_eq!(tui.notifications, Notifications::Enabled(false));
+    }
+
     #[test]
     fn test_sandbox_config_parsing() {
         let sandbox_full_access = r#"

codex-rs/core/src/config_types.rs

@@ -94,6 +94,7 @@ impl Default for Notifications {
 pub struct Tui {
     /// Enable desktop notifications from the TUI when the terminal is unfocused.
     /// Defaults to `false`.
+    #[serde(default)]
     pub notifications: Notifications,
 }