feat: 完成错误原因合同与付款就绪提示（09-05-error-explainability-payment-readiness）

server/src/routes/purchaseRequests.js

@@ -1,4 +1,5 @@
 const { z } = require("zod");
+const { buildBusinessValidationError } = require("../utils/businessValidationError");
 
 const { AppError } = require("../utils/AppError");
 const { buildAuditLogData } = require("../utils/audit");
@@ -2400,6 +2401,7 @@ async function purchaseRequestRoutes(fastify, options = {}) {
           throw buildValidationError({ file: "multipart_limit_exceeded" });
         }
         if (error instanceof PurchaseAttachmentPolicyError) {
+          if (error.reasonCode === "unsupported_type") throw buildBusinessValidationError("purchase_attachment.unsupported_type", error.details ?? { file: error.reasonCode });
           throw buildValidationError(error.details ?? { file: error.reasonCode });
         }
         throw error;