fix(entity): 修复文章和页面附件字段更新策略问题

- 为 CmsArticle 的 files 字段添加忽略更新策略，支持传 null 或空字符串清空附件
- 为 CmsPage 的 attachments 字段添加忽略更新策略，支持传 null 或空字符串清空附件
- 更新字段注释，明确附件字段可以通过空值清空内容
- 引入 FieldStrategy 枚举以支持字段更新策略配置

src/main/java/com/gxwebsoft/cms/entity/CmsArticle.java

@@ -1,5 +1,6 @@
 package com.gxwebsoft.cms.entity;
 
+import com.baomidou.mybatisplus.annotation.FieldStrategy;
 import com.baomidou.mybatisplus.annotation.IdType;
 import com.baomidou.mybatisplus.annotation.TableField;
 import com.baomidou.mybatisplus.annotation.TableId;
@@ -135,7 +136,8 @@ public class CmsArticle implements Serializable {
     @Schema(description = "发布来源客户端 (APP、H5、小程序等)")
     private String platform;
 
-    @Schema(description = "文章附件")
+    @Schema(description = "文章附件(JSON 数组字符串，元素含 name/url/size/ext/sort)；传 null 或空字符串可清空")
+    @TableField(updateStrategy = FieldStrategy.IGNORED)
     private String files;
 
     @Schema(description = "视频地址")

src/main/java/com/gxwebsoft/cms/entity/CmsPage.java

@@ -1,6 +1,7 @@
 package com.gxwebsoft.cms.entity;
 
 import com.baomidou.mybatisplus.annotation.IdType;
+import com.baomidou.mybatisplus.annotation.FieldStrategy;
 import com.baomidou.mybatisplus.annotation.TableField;
 import com.baomidou.mybatisplus.annotation.TableId;
 import com.baomidou.mybatisplus.annotation.TableLogic;
@@ -64,7 +65,8 @@ public class CmsPage implements Serializable {
     @Schema(description = "备注")
     private String comments;
 
-    @Schema(description = "页面附件(JSON 数组字符串，元素含 name/url/size/ext/sort)")
+    @Schema(description = "页面附件(JSON 数组字符串，元素含 name/url/size/ext/sort)；传 null 或空字符串可清空")
+    @TableField(updateStrategy = FieldStrategy.IGNORED)
     private String attachments;
 
     @Schema(description = "阅读量")