Route WebRTC sideband joins to the Realtime API (#35830)

## What changed

- Use `https://api.openai.com/v1` for WebRTC sideband websocket joins instead of deriving the URL from the model provider.
- Keep `experimental_realtime_ws_base_url` as an explicit sideband override for local development and tests.
- Exclude provider query parameters when building sideband URLs.

## Testing

- Add coverage that a frameless sideband join uses `wss://api.openai.com/v1/live/rtc_test` even when the provider points at the ChatGPT backend.
- Update the sideband retry end-to-end test to inject its mock server URL through the new override.

GitOrigin-RevId: 5d3d2cd69905ba04143010ced36a22712825b563

codex-rs/codex-api/src/endpoint/realtime_websocket/methods.rs

@@ -55,6 +55,7 @@ use tungstenite::protocol::WebSocketConfig;
 use url::Url;
 
 const REALTIME_WIRE_LOG_TARGET: &str = "codex_api::realtime_websocket::wire";
+const OPENAI_REALTIME_API_BASE_URL: &str = "https://api.openai.com/v1";
 
 struct WsStream {
     tx_command: mpsc::Sender<WsCommand>,
@@ -696,11 +697,21 @@ fn contains_transcript_entry(entries: &[RealtimeTranscriptEntry], role: &str, te
 
 pub struct RealtimeWebsocketClient {
     provider: Provider,
+    webrtc_sideband_base_url: String,
 }
 
 impl RealtimeWebsocketClient {
     pub fn new(provider: Provider) -> Self {
-        Self { provider }
+        Self {
+            provider,
+            webrtc_sideband_base_url: OPENAI_REALTIME_API_BASE_URL.to_string(),
+        }
+    }
+
+    /// Overrides the direct WebRTC sideband URL for local development and tests.
+    pub fn with_webrtc_sideband_base_url(mut self, base_url: String) -> Self {
+        self.webrtc_sideband_base_url = base_url;
+        self
     }
 
     pub async fn connect(
@@ -775,13 +786,7 @@ impl RealtimeWebsocketClient {
     ) -> Result<RealtimeWebsocketConnection, ApiError> {
         // Keep the parser/session query shaping from standalone realtime while replacing the model
         // query with a call_id join onto an existing WebRTC session.
-        let ws_url = websocket_url_from_api_url_for_call(
-            self.provider.base_url.as_str(),
-            self.provider.query_params.as_ref(),
-            config.event_parser,
-            config.session_mode,
-            call_id,
-        )?;
+        let ws_url = self.webrtc_sideband_url(config.event_parser, config.session_mode, call_id)?;
         self.connect_realtime_websocket_url(
             ws_url,
             config,
@@ -792,6 +797,21 @@ impl RealtimeWebsocketClient {
         .await
     }
 
+    fn webrtc_sideband_url(
+        &self,
+        event_parser: RealtimeEventParser,
+        session_mode: RealtimeSessionMode,
+        call_id: &str,
+    ) -> Result<Url, ApiError> {
+        websocket_url_from_api_url_for_call(
+            self.webrtc_sideband_base_url.as_str(),
+            /*query_params*/ None,
+            event_parser,
+            session_mode,
+            call_id,
+        )
+    }
+
     async fn connect_realtime_websocket_url(
         &self,
         ws_url: Url,
@@ -1013,6 +1033,7 @@ fn normalize_realtime_path(url: &mut Url, event_parser: RealtimeEventParser) {
 mod tests {
     use super::*;
     use crate::endpoint::realtime_websocket::protocol::RealtimeTranscriptEntry;
+    use crate::provider::RetryConfig;
     use codex_protocol::protocol::RealtimeHandoffRequested;
     use codex_protocol::protocol::RealtimeInputAudioSpeechStarted;
     use codex_protocol::protocol::RealtimeNoopRequested;
@@ -1780,6 +1801,34 @@ mod tests {
         );
     }
 
+    #[test]
+    fn webrtc_frameless_sideband_ignores_provider_base_url() {
+        let client = RealtimeWebsocketClient::new(Provider {
+            name: "chatgpt".to_string(),
+            base_url: "https://chatgpt.com/backend-api/codex".to_string(),
+            query_params: None,
+            headers: HeaderMap::new(),
+            retry: RetryConfig {
+                max_attempts: 0,
+                base_delay: Duration::ZERO,
+                retry_429: false,
+                retry_5xx: false,
+                retry_transport: false,
+            },
+            stream_idle_timeout: Duration::from_secs(5),
+        });
+
+        let url = client
+            .webrtc_sideband_url(
+                RealtimeEventParser::FramelessBidi,
+                RealtimeSessionMode::Conversational,
+                "rtc_test",
+            )
+            .expect("build ws url");
+
+        assert_eq!(url.as_str(), "wss://api.openai.com/v1/live/rtc_test");
+    }
+
     #[tokio::test]
     async fn e2e_connect_and_exchange_events_against_mock_ws_server() {
         let listener = TcpListener::bind("127.0.0.1:0").await.expect("bind");