ignore deltas in codex_delegate (#6208)

ignore legacy deltas in codex-delegate to avoid this
[issue](https://github.com/openai/codex/pull/6202).

codex-rs/core/tests/suite/codex_delegate.rs

@@ -8,6 +8,8 @@ use core_test_support::responses::ev_apply_patch_function_call;
 use core_test_support::responses::ev_assistant_message;
 use core_test_support::responses::ev_completed;
 use core_test_support::responses::ev_function_call;
+use core_test_support::responses::ev_reasoning_item_added;
+use core_test_support::responses::ev_reasoning_summary_text_delta;
 use core_test_support::responses::ev_response_created;
 use core_test_support::responses::mount_sse_sequence;
 use core_test_support::responses::sse;
@@ -15,6 +17,7 @@ use core_test_support::responses::start_mock_server;
 use core_test_support::skip_if_no_network;
 use core_test_support::test_codex::test_codex;
 use core_test_support::wait_for_event;
+use pretty_assertions::assert_eq;
 
 /// Delegate should surface ExecApprovalRequest from sub-agent and proceed
 /// after parent submits an approval decision.
@@ -171,3 +174,52 @@ async fn codex_delegate_forwards_patch_approval_and_proceeds_on_decision() {
     .await;
     wait_for_event(&test.codex, |ev| matches!(ev, EventMsg::TaskComplete(_))).await;
 }
+
+#[tokio::test(flavor = "multi_thread", worker_threads = 2)]
+async fn codex_delegate_ignores_legacy_deltas() {
+    skip_if_no_network!();
+
+    // Single response with reasoning summary deltas.
+    let sse_stream = sse(vec![
+        ev_response_created("resp-1"),
+        ev_reasoning_item_added("reason-1", &["initial"]),
+        ev_reasoning_summary_text_delta("think-1"),
+        ev_completed("resp-1"),
+    ]);
+
+    let server = start_mock_server().await;
+    mount_sse_sequence(&server, vec![sse_stream]).await;
+
+    let mut builder = test_codex();
+    let test = builder.build(&server).await.expect("build test codex");
+
+    // Kick off review (delegated).
+    test.codex
+        .submit(Op::Review {
+            review_request: ReviewRequest {
+                prompt: "Please review".to_string(),
+                user_facing_hint: "review".to_string(),
+            },
+        })
+        .await
+        .expect("submit review");
+
+    let mut reasoning_delta_count = 0;
+    let mut legacy_reasoning_delta_count = 0;
+
+    loop {
+        let ev = wait_for_event(&test.codex, |_| true).await;
+        match ev {
+            EventMsg::ReasoningContentDelta(_) => reasoning_delta_count += 1,
+            EventMsg::AgentReasoningDelta(_) => legacy_reasoning_delta_count += 1,
+            EventMsg::TaskComplete(_) => break,
+            _ => {}
+        }
+    }
+
+    assert_eq!(reasoning_delta_count, 1, "expected one new reasoning delta");
+    assert_eq!(
+        legacy_reasoning_delta_count, 1,
+        "expected one legacy reasoning delta"
+    );
+}