Route realtime V3 handoffs by response channel (#33903)

## What changed

- Add `codexResponseHandoffMode` to `thread/realtime/start` for V3 sessions, with `thinking` as the default and `commentary` and `bemTags` routing modes.
- Map BEM `analysis` and `commentary` output to the commentary channel and `final` output to the speakable channel, while preserving the original BEM envelope. Unrecognized output falls back to speakable.
- Mark explicit speech appends as speakable and stop adding the legacy `"Agent Final Message"` prefix to V3 handoffs. V1 and V2 continue to ignore the new setting.

## Testing

- Cover channel selection for every routing mode, streamed BEM header parsing, fallback behavior, explicit speech, and V1 compatibility.

GitOrigin-RevId: 1974578257488f2914b252c9c1990ba38831c96b

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

@@ -9,6 +9,7 @@ use crate::endpoint::realtime_websocket::methods_frameless_bidi::context_append_
 use crate::endpoint::realtime_websocket::methods_frameless_bidi::delegation_context_append_message as frameless_delegation_context_append_message;
 use crate::endpoint::realtime_websocket::methods_frameless_bidi::session_context_append_message as frameless_session_context_append_message;
 use crate::endpoint::realtime_websocket::protocol::RealtimeAudioFrame;
+use crate::endpoint::realtime_websocket::protocol::RealtimeContextAppendChannel;
 use crate::endpoint::realtime_websocket::protocol::RealtimeEvent;
 use crate::endpoint::realtime_websocket::protocol::RealtimeEventParser;
 use crate::endpoint::realtime_websocket::protocol::RealtimeOutboundMessage;
@@ -211,6 +212,7 @@ pub struct RealtimeWebsocketWriter {
     stream: Arc<WsStream>,
     is_closed: Arc<AtomicBool>,
     event_parser: RealtimeEventParser,
+    context_append_channel: Option<RealtimeContextAppendChannel>,
 }
 
 #[derive(Clone)]
@@ -281,6 +283,7 @@ impl RealtimeWebsocketConnection {
                 stream: Arc::clone(&stream),
                 is_closed: Arc::clone(&is_closed),
                 event_parser,
+                context_append_channel: None,
             },
             events: RealtimeWebsocketEvents {
                 rx_message,
@@ -294,6 +297,11 @@ impl RealtimeWebsocketConnection {
 }
 
 impl RealtimeWebsocketWriter {
+    pub fn with_context_append_channel(mut self, channel: RealtimeContextAppendChannel) -> Self {
+        self.context_append_channel = Some(channel);
+        self
+    }
+
     pub async fn send_audio_frame(&self, frame: RealtimeAudioFrame) -> Result<(), ApiError> {
         let message = match self.event_parser {
             RealtimeEventParser::V1 | RealtimeEventParser::RealtimeV2 => {
@@ -315,6 +323,7 @@ impl RealtimeWebsocketWriter {
             self.event_parser,
             text,
             role,
+            self.context_append_channel,
         ))
         .await
     }
@@ -328,6 +337,7 @@ impl RealtimeWebsocketWriter {
             self.event_parser,
             handoff_id,
             output_text,
+            self.context_append_channel,
         ))
         .await
     }
@@ -341,6 +351,7 @@ impl RealtimeWebsocketWriter {
             self.event_parser,
             handoff_id,
             output_text,
+            self.context_append_channel,
         ))
         .await
     }
@@ -354,6 +365,7 @@ impl RealtimeWebsocketWriter {
             self.event_parser,
             call_id,
             output_text,
+            self.context_append_channel,
         ))
         .await
     }
@@ -413,6 +425,7 @@ impl RealtimeWebsocketWriter {
         match message {
             RealtimeOutboundMessage::DelegationContextAppend {
                 delegation_item_id,
+                channel,
                 content,
             } => {
                 if let Some(content) = content.first() {
@@ -420,17 +433,20 @@ impl RealtimeWebsocketWriter {
                         self.send_json_frame(&frameless_delegation_context_append_message(
                             delegation_item_id.clone(),
                             chunk,
+                            *channel,
                         ))
                         .await?;
                     }
                     return Ok(());
                 }
             }
-            RealtimeOutboundMessage::SessionContextAppend { content } => {
+            RealtimeOutboundMessage::SessionContextAppend { channel, content } => {
                 if let Some(content) = content.first() {
                     for chunk in context_append_chunks(&content.text) {
-                        self.send_json_frame(&frameless_session_context_append_message(chunk))
-                            .await?;
+                        self.send_json_frame(&frameless_session_context_append_message(
+                            chunk, *channel,
+                        ))
+                        .await?;
                     }
                     return Ok(());
                 }