Native lpad and rpad fail when the string argument evaluates to a scalar, such as a scalar subquery inside IF #7245

Describe the bug

lpad and rpad fail natively when their string argument references a column but evaluates to a scalar for the whole batch. CometStringLPad and CometStringRPad only keep a Literal string on Spark, so IF(id > 0, (SELECT max(s) FROM strs), s) is serialized. When every row in the batch takes the subquery branch, the conditional returns a scalar, and the native read_side_padding kernel only accepts an array as its first argument:

org.apache.comet.CometNativeException: Unsupported arguments [Scalar(Utf8("cd")), Scalar(Int32(5)), Scalar(Utf8("0"))] for function rpad/lpad/read_side_padding.

Spark returns the padded value. Default configs are enough to hit this.

Steps to reproduce

Seq((1, "ab"), (2, "cd")).toDF("id", "s").repartition(1).write.parquet("/tmp/strs")
spark.read.parquet("/tmp/strs").createOrReplaceTempView("strs")
sql("SELECT id, lpad(IF(id > 0, (SELECT max(s) FROM strs), s), 5, '0') FROM strs").collect()

rpad, and CASE WHEN id > 0 THEN (SELECT max(s) FROM strs) ELSE s END as the string argument, fail the same way.

Reproduced on main at c4a0b1f with the default Spark 4.1 profile.

Expected behavior

Spark's rows: 000cd for both rows with lpad, and cd000 with rpad.

Additional context

This is the padding half of the scalar-argument problem in #3336. #3336 says hour, minute, second and unix_timestamp need ConstantFolding disabled to fail, but a plain scalar subquery reaches them with default configs too. SELECT hour((SELECT max(ts) FROM t)) FROM t fails with hour(scalar) should be fold in Spark JVM side., and unix_timestamp fails the same way. #5889 carries fixes for both halves that don't depend on its struct support.