Math.abs(Integer.MIN_VALUE) will overflow and return a negative value

**Describe the bug**

[Substr.java](https://github.com/apache/geaflow/blob/main/geaflow/geaflow-dsl/geaflow-dsl-plan/src/main/java/org/apache/geaflow/dsl/udf/table/string/Substr.java#L34-L36)

```java
if ((Math.abs(pos) > str.length())) {
return str;
}
```
**Expected behavior**

Math.abs(Integer.MIN_VALUE) will overflow
return a negative value

Start by reading geaflow/geaflow-dsl/geaflow-dsl-plan/src/main/java/org/apache/geaflow/dsl/udf/table/string/Substr.java at lines 34-36, focusing on how the position bound is checked. Reproduce the Integer.MIN_VALUE case and verify that the bounds check no longer produces an incorrect negative result while existing substring behavior remains unchanged.

Written by the indexing model from the issue text.