Add WebRTC transport to realtime start (#16960)

Adds WebRTC startup to the experimental app-server
`thread/realtime/start` method with an optional transport enum. The
websocket path remains the default; WebRTC offers create the realtime
session through the shared start flow and emit the answer SDP via
`thread/realtime/sdp`.

---------

Co-authored-by: Codex <noreply@openai.com>

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

@@ -3854,6 +3854,21 @@ pub struct ThreadRealtimeStartParams {
     pub prompt: String,
     #[ts(optional = nullable)]
     pub session_id: Option<String>,
+    #[ts(optional = nullable)]
+    pub transport: Option<ThreadRealtimeStartTransport>,
+}
+
+/// EXPERIMENTAL - transport used by thread realtime.
+#[derive(Serialize, Deserialize, Debug, Clone, PartialEq, JsonSchema, TS)]
+#[serde(tag = "type", rename_all = "camelCase")]
+#[ts(export_to = "v2/", tag = "type")]
+pub enum ThreadRealtimeStartTransport {
+    Websocket,
+    Webrtc {
+        /// SDP offer generated by a WebRTC RTCPeerConnection after configuring audio and the
+        /// realtime events data channel.
+        sdp: String,
+    },
 }
 
 /// EXPERIMENTAL - response for starting thread realtime.
@@ -3945,6 +3960,15 @@ pub struct ThreadRealtimeOutputAudioDeltaNotification {
     pub audio: ThreadRealtimeAudioChunk,
 }
 
+/// EXPERIMENTAL - emitted with the remote SDP for a WebRTC realtime session.
+#[derive(Serialize, Deserialize, Debug, Clone, PartialEq, JsonSchema, TS)]
+#[serde(rename_all = "camelCase")]
+#[ts(export_to = "v2/")]
+pub struct ThreadRealtimeSdpNotification {
+    pub thread_id: String,
+    pub sdp: String,
+}
+
 /// EXPERIMENTAL - emitted when thread realtime encounters an error.
 #[derive(Serialize, Deserialize, Debug, Clone, PartialEq, JsonSchema, TS)]
 #[serde(rename_all = "camelCase")]