Add notifier tests (#4064)

Proposal:
1. Use anyhow for tests and avoid unwrap
2. Extract a helper for starting a test instance of codex

codex-rs/core/src/user_notification.rs

@@ -1,4 +1,45 @@
 use serde::Serialize;
+use tracing::error;
+use tracing::warn;
+
+#[derive(Debug, Default)]
+pub(crate) struct UserNotifier {
+    notify_command: Option<Vec<String>>,
+}
+
+impl UserNotifier {
+    pub(crate) fn notify(&self, notification: &UserNotification) {
+        if let Some(notify_command) = &self.notify_command
+            && !notify_command.is_empty()
+        {
+            self.invoke_notify(notify_command, notification)
+        }
+    }
+
+    fn invoke_notify(&self, notify_command: &[String], notification: &UserNotification) {
+        let Ok(json) = serde_json::to_string(&notification) else {
+            error!("failed to serialise notification payload");
+            return;
+        };
+
+        let mut command = std::process::Command::new(&notify_command[0]);
+        if notify_command.len() > 1 {
+            command.args(&notify_command[1..]);
+        }
+        command.arg(json);
+
+        // Fire-and-forget – we do not wait for completion.
+        if let Err(e) = command.spawn() {
+            warn!("failed to spawn notifier '{}': {e}", notify_command[0]);
+        }
+    }
+
+    pub(crate) fn new(notify: Option<Vec<String>>) -> Self {
+        Self {
+            notify_command: notify,
+        }
+    }
+}
 
 /// User can configure a program that will receive notifications. Each
 /// notification is serialized as JSON and passed as an argument to the