fix(openai): strip unsupported passthrough fields

backend/internal/service/openai_gateway_service.go

@@ -5454,7 +5454,8 @@ func extractOpenAIRequestMetaFromBody(body []byte) (model string, stream bool, p
 }
 
 // normalizeOpenAIPassthroughOAuthBody 将透传 OAuth 请求体收敛为旧链路关键行为：
-// 1) store=false 2) 非 compact 保持 stream=true；compact 强制 stream=false
+// 1) 删除 ChatGPT internal API 不支持的顶层 Responses 参数
+// 2) store=false 3) 非 compact 保持 stream=true；compact 强制 stream=false
 func normalizeOpenAIPassthroughOAuthBody(body []byte, compact bool) ([]byte, bool, error) {
 	if len(body) == 0 {
 		return body, false, nil
@@ -5463,6 +5464,18 @@ func normalizeOpenAIPassthroughOAuthBody(body []byte, compact bool) ([]byte, boo
 	normalized := body
 	changed := false
 
+	for _, field := range openAIChatGPTInternalUnsupportedFields {
+		if value := gjson.GetBytes(normalized, field); !value.Exists() {
+			continue
+		}
+		next, err := sjson.DeleteBytes(normalized, field)
+		if err != nil {
+			return body, false, fmt.Errorf("normalize passthrough body delete %s: %w", field, err)
+		}
+		normalized = next
+		changed = true
+	}
+
 	if compact {
 		if store := gjson.GetBytes(normalized, "store"); store.Exists() {
 			next, err := sjson.DeleteBytes(normalized, "store")