Add item streaming events (#5546)

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

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

codex-rs/exec/src/event_processor_with_human_output.rs

@@ -519,6 +519,9 @@ impl EventProcessor for EventProcessorWithHumanOutput {
             | EventMsg::AgentReasoningRawContentDelta(_)
             | EventMsg::ItemStarted(_)
             | EventMsg::ItemCompleted(_)
+            | EventMsg::AgentMessageContentDelta(_)
+            | EventMsg::ReasoningContentDelta(_)
+            | EventMsg::ReasoningRawContentDelta(_)
             | EventMsg::UndoCompleted(_)
             | EventMsg::UndoStarted(_) => {}
         }