fix(backend): getOAuth2Url 用 String 重载的 URLEncoder.encode，兼容 Java 8 编译

cfc-backend/src/main/java/com/etotem/cfc/service/WechatService.java

@@ -14,6 +14,7 @@ import org.springframework.web.client.RestTemplate;
 import com.etotem.cfc.service.api.WechatServiceInterface;
 
 import javax.annotation.PostConstruct;
+import java.io.UnsupportedEncodingException;
 import java.net.URLEncoder;
 import java.nio.charset.StandardCharsets;
 import java.util.Base64;
@@ -141,7 +142,14 @@ public class WechatService implements WechatServiceInterface {
      * 获取公众号OAuth2授权URL
      */
     public String getOAuth2Url(String redirectUri) {
-        String encodedUri = URLEncoder.encode(redirectUri, StandardCharsets.UTF_8);
+        String encodedUri;
+        try {
+            encodedUri = URLEncoder.encode(redirectUri, "UTF-8");
+        } catch (UnsupportedEncodingException e) {
+            // UTF-8 在 JVM 中始终可用，此异常实际不会发生
+            log.error("OAuth2 redirectUri 编码失败", e);
+            encodedUri = redirectUri;
+        }
         return String.format(
             "https://open.weixin.qq.com/connect/oauth2/authorize?appid=%s&redirect_uri=%s&response_type=code&scope=snsapi_base&state=STATE#wechat_redirect",
             appid, encodedUri);