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/resume_picker_transcript_preview_tests.rs

@@ -41,6 +41,7 @@ fn rollout_agent_message(text: &str) -> RolloutItem {
         message: text.to_string(),
         phase: None,
         memory_citation: None,
+        delivery: None,
     }))
 }
 
@@ -258,6 +259,7 @@ fn transcript_preview_reverse_scan_stops_before_older_items() {
         text: String::from("one\ntwo\nthree\nfour\nfive\nsix"),
         phase: None,
         memory_citation: None,
+        delivery: None,
     };
     let mut lines = Vec::new();