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/realtime.rs

@@ -96,6 +96,11 @@ pub struct ThreadRealtimeStartParams {
     /// Set to false to start without Codex's startup context. Omitted or null includes it.
     #[ts(optional = nullable)]
     pub include_startup_context: Option<bool>,
+    /// Adds complete role-bearing text items to the initial Frameless Bidi session history.
+    /// This is only supported by realtime V3 and is sent during session startup. Requests are
+    /// limited to 128 items and 8,192 estimated text tokens in total.
+    #[ts(optional = nullable)]
+    pub initial_items: Option<Vec<ThreadRealtimeInitialItem>>,
     #[serde(
         default,
         deserialize_with = "crate::protocol::serde_helpers::deserialize_double_option",
@@ -115,6 +120,15 @@ pub struct ThreadRealtimeStartParams {
     pub voice: Option<RealtimeVoice>,
 }
 
+/// EXPERIMENTAL - role-bearing text item included when a realtime V3 session starts.
+#[derive(Serialize, Deserialize, Debug, Clone, PartialEq, JsonSchema, TS)]
+#[serde(rename_all = "camelCase")]
+#[ts(export_to = "v2/")]
+pub struct ThreadRealtimeInitialItem {
+    pub role: ConversationTextRole,
+    pub text: String,
+}
+
 /// EXPERIMENTAL - transport used by thread realtime.
 #[derive(Serialize, Deserialize, Debug, Clone, PartialEq, JsonSchema, TS)]
 #[serde(tag = "type", rename_all = "camelCase")]

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);
+}