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/app-server-protocol/schema/json/codex_app_server_protocol.v2.schemas.json

@@ -313,6 +313,12 @@
       },
       "type": "object"
     },
+    "AgentMessageDelivery": {
+      "enum": [
+        "async"
+      ],
+      "type": "string"
+    },
     "AgentMessageDeltaNotification": {
       "$schema": "http://json-schema.org/draft-07/schema#",
       "properties": {
@@ -17551,6 +17557,17 @@
         },
         {
           "properties": {
+            "delivery": {
+              "anyOf": [
+                {
+                  "$ref": "#/definitions/AgentMessageDelivery"
+                },
+                {
+                  "type": "null"
+                }
+              ],
+              "default": null
+            },
             "id": {
               "type": "string"
             },