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/rollout/policy.rs

@@ -77,6 +77,9 @@ pub(crate) fn should_persist_event_msg(ev: &EventMsg) -> bool {
         | EventMsg::ViewImageToolCall(_)
         | EventMsg::DeprecationNotice(_)
         | EventMsg::ItemStarted(_)
-        | EventMsg::ItemCompleted(_) => false,
+        | EventMsg::ItemCompleted(_)
+        | EventMsg::AgentMessageContentDelta(_)
+        | EventMsg::ReasoningContentDelta(_)
+        | EventMsg::ReasoningRawContentDelta(_) => false,
     }
 }