Add item streaming events (#5546)

Adds AgentMessageContentDelta, ReasoningContentDelta,
ReasoningRawContentDelta item streaming events while maintaining
compatibility for old events.

---------

Co-authored-by: Owen Lin <owen@openai.com>

codex-rs/core/src/tasks/review.rs

@@ -1,11 +1,13 @@
 use std::sync::Arc;
 
 use async_trait::async_trait;
+use codex_protocol::items::TurnItem;
 use codex_protocol::models::ContentItem;
 use codex_protocol::models::ResponseItem;
 use codex_protocol::protocol::Event;
 use codex_protocol::protocol::EventMsg;
 use codex_protocol::protocol::ExitedReviewModeEvent;
+use codex_protocol::protocol::ItemCompletedEvent;
 use codex_protocol::protocol::ReviewOutputEvent;
 use tokio_util::sync::CancellationToken;
 
@@ -109,6 +111,13 @@ async fn process_review_events(
                 }
                 prev_agent_message = Some(event);
             }
+            // Suppress ItemCompleted for assistant messages: forwarding it would
+            // trigger legacy AgentMessage via as_legacy_events(), which this
+            // review flow intentionally hides in favor of structured output.
+            EventMsg::ItemCompleted(ItemCompletedEvent {
+                item: TurnItem::AgentMessage(_),
+                ..
+            }) => {}
             EventMsg::TaskComplete(task_complete) => {
                 // Parse review output from the last agent message (if present).
                 let out = task_complete