Native engine panics on all-scalar inputs for Substring and StringSpace

[andygrove]

Description

When Spark's ConstantFolding optimizer rule is disabled, all-scalar (literal-only) expressions reach Comet's native engine. The native engine panics with:

• Substring(scalar) should be fold in Spark JVM side.
• StringSpace(scalar) should be fold in Spark JVM side.

How to Reproduce

SET spark.sql.optimizer.excludedRules=org.apache.spark.sql.catalyst.optimizer.ConstantFolding;
SELECT substring('hello world', 1, 5);
SELECT space(5);
SELECT left('hello', 3);

Expected Behavior

The native engine should handle scalar inputs gracefully — either by computing the correct result or by falling back to Spark, not by panicking.

Affected Expressions

• substring() / left()
• space()

[kazantsev-maksim]

I'd like to work on it.