fix(responses): reject invalid SSE data JSON

Guard the openai-response streaming path against truncated/invalid SSE data payloads by validating data: JSON before forwarding; surface a 502 terminal error instead of letting clients crash with JSON parse errors.

sdk/api/handlers/handlers.go

@@ -716,6 +716,12 @@ func (h *BaseAPIHandler) ExecuteStreamWithAuthManager(ctx context.Context, handl
 					return
 				}
 				if len(chunk.Payload) > 0 {
+					if handlerType == "openai-response" {
+						if err := validateSSEDataJSON(chunk.Payload); err != nil {
+							_ = sendErr(&interfaces.ErrorMessage{StatusCode: http.StatusBadGateway, Error: err})
+							return
+						}
+					}
 					sentPayload = true
 					if okSendData := sendData(cloneBytes(chunk.Payload)); !okSendData {
 						return
@@ -727,6 +733,35 @@ func (h *BaseAPIHandler) ExecuteStreamWithAuthManager(ctx context.Context, handl
 	return dataChan, upstreamHeaders, errChan
 }
 
+func validateSSEDataJSON(chunk []byte) error {
+	for _, line := range bytes.Split(chunk, []byte("\n")) {
+		line = bytes.TrimSpace(line)
+		if len(line) == 0 {
+			continue
+		}
+		if !bytes.HasPrefix(line, []byte("data:")) {
+			continue
+		}
+		data := bytes.TrimSpace(line[5:])
+		if len(data) == 0 {
+			continue
+		}
+		if bytes.Equal(data, []byte("[DONE]")) {
+			continue
+		}
+		if json.Valid(data) {
+			continue
+		}
+		const max = 512
+		preview := data
+		if len(preview) > max {
+			preview = preview[:max]
+		}
+		return fmt.Errorf("invalid SSE data JSON (len=%d): %q", len(data), preview)
+	}
+	return nil
+}
+
 func statusFromError(err error) int {
 	if err == nil {
 		return 0