fix: make SMS retry side effects idempotent

api/src/billing/billing.service.spec.ts

@@ -18,7 +18,8 @@ function createPrismaMock() {
         return Promise.resolve({ count: 1 });
       }),
       update: jest.fn().mockImplementation(({ data }) => {
-        if (data.balanceCents !== undefined) accountState.balanceCents = data.balanceCents;
+        if (data.balanceCents?.increment !== undefined) accountState.balanceCents += data.balanceCents.increment;
+        else if (data.balanceCents !== undefined) accountState.balanceCents = data.balanceCents;
         if (data.creditCents !== undefined) accountState.creditCents = data.creditCents;
         return Promise.resolve({ ...accountState });
       }),
@@ -26,6 +27,7 @@ function createPrismaMock() {
     accountTransaction: {
       findMany: jest.fn(),
       findFirst: jest.fn(),
+      findUnique: jest.fn().mockResolvedValue(null),
       create: jest.fn().mockImplementation(({ data }) => Promise.resolve({ id: `tx-${data.transactionType}`, ...data })),
     },
     rechargeOrder: {
@@ -125,7 +127,7 @@ describe('BillingService', () => {
     expect(order).toEqual(expect.objectContaining({ amountCents: 500, status: 'paid' }));
     expect(prisma.tenantAccount.update).toHaveBeenCalledWith({
       where: { tenantId: 'tenant-1' },
-      data: { balanceCents: 1500 },
+      data: { balanceCents: { increment: 500 } },
     });
     expect(prisma.accountTransaction.create).toHaveBeenCalledWith({
       data: expect.objectContaining({
@@ -302,6 +304,46 @@ describe('BillingService', () => {
     expect(prisma.accountState).toEqual(expect.objectContaining({ balanceCents: 890 }));
   });
 
+  it('serializes and replays concurrent refunds with one balance mutation', async () => {
+    const prisma = createPrismaMock();
+    let transactionChain = Promise.resolve<unknown>(undefined);
+    let persistedTransaction: Record<string, unknown> | null = null;
+    prisma.$transaction.mockImplementation((callback) => {
+      const run = transactionChain.then(() => callback(prisma));
+      transactionChain = run.then(() => undefined, () => undefined);
+      return run;
+    });
+    prisma.accountTransaction.findUnique.mockImplementation(() => Promise.resolve(persistedTransaction));
+    prisma.accountTransaction.create.mockImplementation(({ data }) => {
+      persistedTransaction = { id: 'tx-refund-once', ...data };
+      return Promise.resolve(persistedTransaction);
+    });
+    const service = new BillingService(prisma as never);
+    const refund = {
+      tenantId: 'tenant-1',
+      amountCents: 1053,
+      idempotencyKey: 'sms-refund:MSG-LONG-RACE',
+      relatedType: 'sms_message_record',
+      relatedId: 'MSG-LONG-RACE',
+      remark: '最终失败退款',
+    };
+
+    const results = await Promise.all([
+      service.refund(refund),
+      service.refund(refund),
+      service.refund(refund),
+    ]);
+
+    expect(results.map((result) => result.id)).toEqual([
+      'tx-refund-once',
+      'tx-refund-once',
+      'tx-refund-once',
+    ]);
+    expect(prisma.tenantAccount.update).toHaveBeenCalledTimes(1);
+    expect(prisma.accountTransaction.create).toHaveBeenCalledTimes(1);
+    expect(prisma.accountState.balanceCents).toBe(2053);
+  });
+
   it('creates SMS billing records linked to message and task identifiers', async () => {
     const prisma = createPrismaMock();
     const service = new BillingService(prisma as never);