Add a realtime delegation acknowledgement control (#36413)

## What changed

- Add the optional `delegationAckFiller` field to `thread/realtime/start`.
- Forward explicit `true` or `false` values to V3 Frameless Bidi session payloads as `delegation.ack_filler`; leave the field absent when unspecified so the Realtime API default is preserved.
- Document that V1 and V2 ignore this setting.

## Testing

- Cover both boolean values in Frameless Bidi serialization and verify the WebRTC V3 session creation payload.

GitOrigin-RevId: 9fe8efd14e42738c3e86e3e9a8af81d0f4521244

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

@@ -72,6 +72,10 @@ pub struct ThreadRealtimeStartParams {
     /// them automatically. Defaults to false.
     #[ts(optional = nullable)]
     pub client_managed_handoffs: Option<bool>,
+    /// Controls whether a realtime V3 delegation produces an acknowledgement filler.
+    /// Omitted values preserve the Realtime API's default behavior.
+    #[ts(optional = nullable)]
+    pub delegation_ack_filler: Option<bool>,
     /// Routes any transcript tail remaining at session end through Codex. Defaults to false.
     /// TODO: Remove this rollout knob once transcript-tail flushing is always enabled.
     #[ts(optional = nullable)]

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

@@ -4731,7 +4731,6 @@ fn realtime_start_omitted_initial_items_remain_none() {
 
     assert_eq!(params.initial_items, None);
 }
-
 #[test]
 fn realtime_start_deserializes_client_handoff_channel_prefixes() {
     let params = serde_json::from_value::<ThreadRealtimeStartParams>(json!({