feat: 增强地址处理逻辑，支持日本地区并优化相关测试用例

background/steps/fill-plus-checkout.js

@@ -12,6 +12,7 @@
     FR: '/fr-address',
     US: '/',
   };
+  const MEIGUODIZHI_SUPPORTED_COUNTRIES = new Set(Object.keys(MEIGUODIZHI_PATH_BY_COUNTRY));
 
   function createPlusCheckoutBillingExecutor(deps = {}) {
     const {
@@ -75,6 +76,9 @@
       if (typeof fetchImpl !== 'function') {
         return null;
       }
+      if (!MEIGUODIZHI_SUPPORTED_COUNTRIES.has(countryCode)) {
+        return null;
+      }
       const path = MEIGUODIZHI_PATH_BY_COUNTRY[countryCode] || MEIGUODIZHI_PATH_BY_COUNTRY.DE;
       const city = normalizeText(fallbackSeed?.fallback?.city || fallbackSeed?.query || '');
       const response = await fetchImpl(MEIGUODIZHI_ADDRESS_ENDPOINT, {