Seed realtime V3 sessions with initial text items (#34067)

## What changed

- Add an optional `initialItems` field to `thread/realtime/start` for seeding
  realtime V3 session history with `user`, `developer`, and `assistant` text.
- Encode the items in Frameless Bidi session bootstrap payloads for websocket
  and WebRTC transports while preserving existing payloads when the list is
  omitted or empty.
- Reject non-empty initial items for V1 and V2, more than 128 items, any item
  over 8,192 estimated tokens, or a combined total over 8,192 estimated tokens.

## Testing

- Cover protocol serialization, app-server forwarding, websocket and WebRTC
  payloads, empty-list compatibility, version checks, and size limits.

GitOrigin-RevId: 09af367f9a07543e3f8d59b7fa5387f932dc5a6a

codex-rs/app-server-protocol/src/protocol/v2/tests.rs

@@ -4375,3 +4375,14 @@ fn realtime_append_text_defaults_role_to_user() {
         }
     );
 }
+
+#[test]
+fn realtime_start_omitted_initial_items_remain_none() {
+    let params = serde_json::from_value::<ThreadRealtimeStartParams>(json!({
+        "threadId": "thread_123",
+        "outputModality": "audio",
+    }))
+    .expect("params should deserialize");
+
+    assert_eq!(params.initial_items, None);
+}