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/exec/src/event_processor_with_human_output_tests.rs

@@ -250,6 +250,7 @@ fn final_message_from_turn_items_uses_latest_agent_message() {
             text: "first".to_string(),
             phase: None,
             memory_citation: None,
+            delivery: None,
         },
         ThreadItem::Plan {
             id: "plan-1".to_string(),
@@ -260,6 +261,7 @@ fn final_message_from_turn_items_uses_latest_agent_message() {
             text: "second".to_string(),
             phase: None,
             memory_citation: None,
+            delivery: None,
         },
     ]);
 
@@ -318,6 +320,7 @@ fn turn_completed_recovers_final_message_from_turn_items() {
                     text: "final answer".to_string(),
                     phase: None,
                     memory_citation: None,
+                    delivery: None,
                 }],
                 status: TurnStatus::Completed,
                 error: None,
@@ -366,6 +369,7 @@ fn turn_completed_overwrites_stale_final_message_from_turn_items() {
                     text: "final answer".to_string(),
                     phase: None,
                     memory_citation: None,
+                    delivery: None,
                 }],
                 status: TurnStatus::Completed,
                 error: None,

codex-rs/exec/src/event_processor_with_jsonl_output_tests.rs

@@ -18,6 +18,7 @@ fn failed_turn_does_not_overwrite_output_last_message_file() {
                 text: "partial answer".to_string(),
                 phase: None,
                 memory_citation: None,
+                delivery: None,
             },
             thread_id: "thread-1".to_string(),
             turn_id: "turn-1".to_string(),

codex-rs/exec/src/lib_tests.rs

@@ -363,6 +363,7 @@ fn turn_items_for_thread_returns_matching_turn_items() {
                     text: "hello".to_string(),
                     phase: None,
                     memory_citation: None,
+                    delivery: None,
                 }],
                 status: codex_app_server_protocol::TurnStatus::Completed,
                 error: None,
@@ -393,6 +394,7 @@ fn turn_items_for_thread_returns_matching_turn_items() {
             text: "hello".to_string(),
             phase: None,
             memory_citation: None,
+            delivery: None,
         }])
     );
     assert_eq!(turn_items_for_thread(&thread, "missing-turn"), None);