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/core/src/thread_manager_tests.rs

@@ -2100,6 +2100,7 @@ fn completed_legacy_event_history_is_not_mid_turn() {
             message: "done".to_string(),
             phase: None,
             memory_citation: None,
+            delivery: None,
         })),
     ]);