Negative-scale decimal cast to timestamp panics #7002

With spark.sql.legacy.allowNegativeScaleOfDecimal=true, CAST(CAST(c * 1E2 AS TIMESTAMP) AS LONG) returns -45600, 0, 12300 in Spark. Comet panics in 10_i128.pow(scale as u32) with attempt to exponentiate with overflow. CometCast.canCastFromDecimal reports the cast as compatible for any scale. The decimal-to-string path already checks that setting.

This predates and is unchanged by #6969; found in review there.