fix command duration display (#1806)

we were always displaying "0ms" before.

<img width="731" height="101" alt="Screenshot 2025-08-02 at 10 51 22 PM"
src="https://github.com/user-attachments/assets/f56814ed-b9a4-4164-9e78-181c60ce19b7"
/>

codex-rs/core/src/protocol.rs

@@ -523,6 +523,8 @@ pub struct ExecCommandEndEvent {
     pub stderr: String,
     /// The command's exit code.
     pub exit_code: i32,
+    /// The duration of the command execution.
+    pub duration: Duration,
 }
 
 #[derive(Debug, Clone, Deserialize, Serialize)]