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

server/tests/unit/utils/businessValidationError.test.js

@@ -0,0 +1,22 @@
+const { ERROR_REASONS, getErrorReason } = require("../../../src/constants/errorReasons");
+const { buildBusinessValidationError } = require("../../../src/utils/businessValidationError");
+
+describe("business validation reason contract", () => {
+  it("preserves status, top-level code and legacy fields without allowing reason override", () => {
+    const error = buildBusinessValidationError("payment_batch.legal_entity_required", {
+      legalEntity: "single_legal_entity_required", reasonCode: "not_registered",
+    }, 409);
+    expect(error).toMatchObject({
+      code: "VALID_001", statusCode: 409, message: "参数校验失败",
+      details: { legalEntity: "single_legal_entity_required", reasonCode: "payment_batch.legal_entity_required" },
+    });
+    expect(getErrorReason(error.details.reasonCode).defaultMessage).toBe("报销人尚未维护所属公司，请联系管理员补充后再安排付款。");
+  });
+
+  it("rejects unknown reasons and protects registered metadata", () => {
+    expect(() => buildBusinessValidationError("payment_batch.typo")).toThrow(TypeError);
+    expect(new Set(ERROR_REASONS.map((reason) => reason.code)).size).toBe(ERROR_REASONS.length);
+    expect(Object.isFrozen(ERROR_REASONS)).toBe(true);
+    expect(ERROR_REASONS.every(Object.isFrozen)).toBe(true);
+  });
+});