fix(网关): 修复工具续链校验与存储策略

完善 function_call_output 续链校验与引用匹配
续链场景强制 store=true，过滤 input 时避免副作用
补充续链判断与过滤相关单元测试

测试: go test ./...

backend/internal/handler/openai_gateway_handler.go

@@ -114,6 +114,26 @@ func (h *OpenAIGatewayHandler) Responses(c *gin.Context) {
 
 	setOpsRequestContext(c, reqModel, reqStream, body)
 
+	// 提前校验 function_call_output 是否具备可关联上下文，避免上游 400。
+	// 要求 previous_response_id，或 input 内存在带 call_id 的 tool_call/function_call，
+	// 或带 id 且与 call_id 匹配的 item_reference。
+	if service.HasFunctionCallOutput(reqBody) {
+		previousResponseID, _ := reqBody["previous_response_id"].(string)
+		if strings.TrimSpace(previousResponseID) == "" && !service.HasToolCallContext(reqBody) {
+			if service.HasFunctionCallOutputMissingCallID(reqBody) {
+				log.Printf("[OpenAI Handler] function_call_output 缺少 call_id: model=%s", reqModel)
+				h.errorResponse(c, http.StatusBadRequest, "invalid_request_error", "function_call_output requires call_id or previous_response_id; if relying on history, ensure store=true and reuse previous_response_id")
+				return
+			}
+			callIDs := service.FunctionCallOutputCallIDs(reqBody)
+			if !service.HasItemReferenceForCallIDs(reqBody, callIDs) {
+				log.Printf("[OpenAI Handler] function_call_output 缺少匹配的 item_reference: model=%s", reqModel)
+				h.errorResponse(c, http.StatusBadRequest, "invalid_request_error", "function_call_output requires item_reference ids matching each call_id, or previous_response_id/tool_call context; if relying on history, ensure store=true and reuse previous_response_id")
+				return
+			}
+		}
+	}
+
 	// Track if we've started streaming (for error handling)
 	streamStarted := false