feat(executor): add upstream disconnect handling for Codex WebSocket sessions

- Introduced `UpstreamDisconnectChan` for Codex WebSocket sessions to notify downstream connections of upstream disconnections.
- Implemented `notifyUpstreamDisconnect` to signal errors and close channels on disconnect events.
- Added integration tests to validate WebSocket session behavior on upstream disconnect.
- Updated OpenAI WebSocket response handlers to properly close connections upon upstream disconnect notifications.

sdk/api/handlers/openai/openai_responses_websocket.go

@@ -56,6 +56,31 @@ func (h *OpenAIResponsesAPIHandler) ResponsesWebsocket(c *gin.Context) {
 	retainResponsesWebsocketToolCaches(downstreamSessionKey)
 	clientIP := websocketClientAddress(c)
 	log.Infof("responses websocket: client connected id=%s remote=%s", passthroughSessionID, clientIP)
+
+	wsDone := make(chan struct{})
+	defer close(wsDone)
+
+	if h != nil && h.AuthManager != nil {
+		if exec, ok := h.AuthManager.Executor("codex"); ok && exec != nil {
+			type upstreamDisconnectSubscriber interface {
+				UpstreamDisconnectChan(sessionID string) <-chan error
+			}
+			if subscriber, ok := exec.(upstreamDisconnectSubscriber); ok && subscriber != nil {
+				disconnectCh := subscriber.UpstreamDisconnectChan(passthroughSessionID)
+				if disconnectCh != nil {
+					go func() {
+						select {
+						case <-wsDone:
+							return
+						case <-disconnectCh:
+							_ = conn.Close()
+						}
+					}()
+				}
+			}
+		}
+	}
+
 	var wsTerminateErr error
 	var wsTimelineLog strings.Builder
 	defer func() {