Decimal casting (#8562)

Signed-off-by: "Nicholas Gates" <nick@nickgates.com>
Signed-off-by: Nicholas Gates <nick@nickgates.com>
Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

Author: gatesn

vortex-array/src/arrays/constant/compute/cast.rs

@@ -25,8 +25,15 @@ mod tests {
     use rstest::rstest;
 
     use crate::IntoArray;
+    use crate::LEGACY_SESSION;
+    use crate::VortexSessionExecute;
     use crate::arrays::ConstantArray;
+    use crate::builtins::ArrayBuiltins;
     use crate::compute::conformance::cast::test_cast_conformance;
+    use crate::dtype::DType;
+    use crate::dtype::DecimalDType;
+    use crate::dtype::Nullability;
+    use crate::scalar::DecimalValue;
     use crate::scalar::Scalar;
 
     #[rstest]
@@ -39,4 +46,22 @@ mod tests {
     fn test_cast_constant_conformance(#[case] array: crate::ArrayRef) {
         test_cast_conformance(&array);
     }
+
+    #[test]
+    fn test_cast_constant_i64_to_decimal() {
+        let target_dtype = DType::Decimal(DecimalDType::new(21, 2), Nullability::NonNullable);
+        let casted = ConstantArray::new(Scalar::from(42i64), 5)
+            .into_array()
+            .cast(target_dtype.clone())
+            .unwrap();
+
+        assert_eq!(casted.dtype(), &target_dtype);
+        let scalar = casted
+            .execute_scalar(0, &mut LEGACY_SESSION.create_execution_ctx())
+            .unwrap();
+        assert_eq!(
+            scalar.as_decimal().decimal_value(),
+            Some(DecimalValue::I128(4200))
+        );
+    }
 }