fix: 添加 Accept 请求头并修正企业发票类型判断

- WebChat Pay v3 API 要求 Accept 请求头
- invoiceType 值为 ORGANIZATION 而非"企业"，修正 buyer type 映射

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

service/src/main/java/com/kym/service/wechat/impl/WechatPayFapiaoService.java

@@ -28,6 +28,7 @@ public class WechatPayFapiaoService {
 
     static {
         JSON_HEADERS = new HttpHeaders();
+        JSON_HEADERS.addHeader("Accept", "application/json");
         JSON_HEADERS.addHeader("Content-Type", "application/json");
     }
 
@@ -117,7 +118,7 @@ public class WechatPayFapiaoService {
         }
 
         var buyerInfo = new HashMap<String, Object>();
-        buyerInfo.put("type", invoice.getInvoiceType() != null && invoice.getInvoiceType().equals("企业")
+        buyerInfo.put("type", "ORGANIZATION".equals(invoice.getInvoiceType())
                 ? "ENTERPRISE" : "INDIVIDUAL");
         buyerInfo.put("name", invoice.getInvoiceTitle());
         if (invoice.getTaxId() != null && !invoice.getTaxId().isBlank()) {