Show better error message when Model class isn't abstract

还没有人认领这个 Issue。

**Describe the Bug**
When you extend the ORM Model to your own Model and forget to set it to abstract, you get an Exception saying
```
Exception in thread "main" java.lang.NullPointerException: Cannot invoke "java.lang.reflect.Field.getType()" because the return value of "org.javawebstack.orm.TableInfo.getField(String)" is null
at org.javawebstack.orm.TableInfo.getIdType(TableInfo.java:282)
at org.javawebstack.orm.TableInfo.analyzeTable(TableInfo.java:95)
at org.javawebstack.orm.TableInfo.constructInfo(TableInfo.java:61)
at org.javawebstack.orm.TableInfo.(TableInfo.java:56)
at org.javawebstack.orm.Repo.(Repo.java:35)
at org.javawebstack.orm.ORM.register(ORM.java:25)
at org.javawebstack.orm.ORM.register(ORM.java:37)

```
**To Reproduce**
Steps to reproduce the behavior:
1. Forget to set your model class to abstract

**Expected behavior**
Maybe show some better error message which indicates that your base model isn't abstract yet.

**Environment Information:**
- Java Language Level 21
- ORM Version 1.0.3-SNAPSHOT

通过注册一个非抽象模型来复现故障，然后从 ORM.register 开始跟踪调用栈，进入 Repo.java 和 TableInfo.java，重点查看报告行附近的 constructInfo、analyzeTable 和 getIdType。检查现有的模型验证和错误处理路径。完成的标准是：这种误用会产生一个清晰的异常，指出基模型必须是抽象的，而不是报告的 NullPointerException。

由索引模型根据 Issue 内容生成。