Add ItemStarted/ItemCompleted events for UserInputItem (#5306)

Adds a new ItemStarted event and delivers UserMessage as the first item
type (more to come).


Renames `InputItem` to `UserInput` considering we're using the `Item`
suffix for actual items.

codex-rs/exec/src/event_processor_with_human_output.rs

@@ -517,6 +517,8 @@ impl EventProcessor for EventProcessorWithHumanOutput {
             EventMsg::AgentMessageDelta(_) => {}
             EventMsg::AgentReasoningDelta(_) => {}
             EventMsg::AgentReasoningRawContentDelta(_) => {}
+            EventMsg::ItemStarted(_) => {}
+            EventMsg::ItemCompleted(_) => {}
         }
         CodexStatus::Running
     }