feat(trade): connect WeChat refund API and callback flow

Co-authored-by: Cursor <cursoragent@cursor.com>

server/dukang-api/src/integrations/wechat/wechat.mock.provider.ts

@@ -88,6 +88,14 @@ export class WechatMockProvider implements IWechatProvider {
     throw new NotImplementedException('FEATURE_DISABLED');
   }
 
+  createDomesticRefund(): never {
+    throw new NotImplementedException('FEATURE_DISABLED');
+  }
+
+  parseRefundNotification(): never {
+    throw new NotImplementedException('FEATURE_DISABLED');
+  }
+
   /** Mock：用普通二维码 PNG 占位，内容含 scene，便于本地联调上传 OSS */
   async getWxaCodeUnlimited(input: WechatWxaCodeUnlimitedInput): Promise<Buffer> {
     const scene = (input.scene ?? '').trim() || 'mock';