refactor(vo): 重构VO类及相关模型，添加Voable接口实现

feat(constant): 添加WebSocket错误码常量
docs(model): 为模型类添加注释
fix(service): 修复ProductUsageService缓存键问题
refactor(converter): 重构字符串转换器，移除EntityStringConverter依赖
feat(tab): 添加ComboBoxUtils工具类，优化下拉框初始化
style: 移除无用导入和字段

common/src/main/java/com/ecep/contract/model/CompanyContract.java

@@ -5,6 +5,7 @@ import java.util.Map;
 import java.util.Objects;
 
 import com.ecep.contract.util.HibernateProxyUtils;
+import com.ecep.contract.vo.CompanyContractVo;
 
 import jakarta.persistence.Column;
 import jakarta.persistence.Entity;
@@ -31,7 +32,7 @@ import lombok.ToString;
 
 })
 @ToString(exclude = { "company", "contract" })
-public class CompanyContract implements IdentityEntity, CompanyBasedEntity, Serializable {
+public class CompanyContract implements IdentityEntity, CompanyBasedEntity, Serializable, Voable<CompanyContractVo> {
     private static final long serialVersionUID = 1L;
     @Id
     @GeneratedValue(strategy = GenerationType.IDENTITY)
@@ -75,4 +76,15 @@ public class CompanyContract implements IdentityEntity, CompanyBasedEntity, Seri
     public final int hashCode() {
         return HibernateProxyUtils.hashCode(this);
     }
+
+    @Override
+    public CompanyContractVo toVo() {
+        CompanyContractVo vo = new CompanyContractVo();
+        vo.setId(getId());
+        vo.setCompanyId(getCompany() != null ? getCompany().getId() : null);
+        vo.setContractId(getContract() != null ? getContract().getId() : null);
+        vo.setContractCode(getContract() != null ? getContract().getCode() : null);
+        vo.setContractName(getContract() != null ? getContract().getName() : null);
+        return vo;
+    }
 }