Add async delivery metadata to agent messages (#39312)

## What changed

- Add an optional `delivery` field to agent message events and app-server
  `agentMessage` items, with `"async"` identifying a user-visible message sent
  without ending the current turn.
- Preserve the marker through legacy event conversion, thread history
  materialization, replay, and generated JSON and TypeScript schemas.
- Require delivery of async agent message completion notifications on the
  in-process app-server transport.

## Testing

- Cover preservation of async delivery metadata in thread history.
- Cover delivery-required classification for async agent messages.

GitOrigin-RevId: f92ba25de1293ee271404badfdf0d117d6530329

codex-rs/tui/src/chatwidget/tests/app_server.rs

@@ -621,6 +621,7 @@ async fn live_app_server_turn_completed_clears_working_status_after_answer_item(
         text: "Yes. What do you need?".to_string(),
         phase: Some(MessagePhase::FinalAnswer),
         memory_citation: None,
+        delivery: None,
     };
     chat.handle_server_notification(
         ServerNotification::ItemCompleted(ItemCompletedNotification {
@@ -1236,6 +1237,7 @@ async fn live_app_server_turn_completion_repairs_dropped_message_deltas() {
         .to_string(),
         phase: Some(MessagePhase::FinalAnswer),
         memory_citation: None,
+        delivery: None,
     }];
     chat.handle_server_notification(
         ServerNotification::TurnCompleted(TurnCompletedNotification {