refactor(common-mail): 使用 MailBuilder 替代 MailUtils

- 新增链式 MailBuilder，统一邮件发送入口
  - 删除 MailUtils，业务代码全部切换为 builder 调用
  - 默认邮件账户改为发送时获取，避免类加载阶段依赖 Bean
  - 自定义发件账号时复制默认配置，避免污染全局 MailAccount
  - 增加收件人、标题和正文参数校验
  - 调整内联图片流关闭时机，发送结束后统一关闭
  - 将 MailAccount 构建逻辑下沉到 MailProperties

ruoyi-modules/ruoyi-workflow/src/main/java/org/dromara/workflow/service/impl/FlwCommonServiceImpl.java

@@ -11,7 +11,7 @@ import org.dromara.common.core.exception.ServiceException;
 import org.dromara.common.core.utils.SpringUtils;
 import org.dromara.common.core.utils.StreamUtils;
 import org.dromara.common.core.utils.StringUtils;
-import org.dromara.common.mail.utils.MailUtils;
+import org.dromara.common.mail.core.MailBuilder;
 import org.dromara.system.api.MessageService;
 import org.dromara.system.api.domain.PushPayloadDTO;
 import org.dromara.system.api.domain.UserDTO;
@@ -122,7 +122,7 @@ public class FlwCommonServiceImpl implements IFlwCommonService {
                             message, null, path
                         ));
                     }
-                    case EMAIL_MESSAGE -> MailUtils.sendText(emails, subject, message);
+                    case EMAIL_MESSAGE -> MailBuilder.of().to(emails).subject(subject).text(message).send();
                     case SMS_MESSAGE -> {
 //                        LinkedHashMap<String, String> map = new LinkedHashMap<>(1);
 //                        // 根据具体短信服务商参数用法传参