Do not attempt to append after response.completed (#11402)

Completed responses are fully done, and new response must be created.

codex-rs/core/src/codex.rs

@@ -4988,6 +4988,7 @@ async fn try_run_sampling_request(
             ResponseEvent::Completed {
                 response_id: _,
                 token_usage,
+                can_append: _,
             } => {
                 if let Some(state) = plan_mode_state.as_mut() {
                     flush_proposed_plan_segments_all(&sess, &turn_context, state).await;