concat_ws with array<string> arguments fails natively instead of flattening like Spark

[peterxcli]

Describe the bug

Spark's ConcatWs accepts both StringType and ArrayType(StringType) children and flattens array arguments: concat_ws(',', array('a','b'), 'c') = a,b,c. CometConcatWs.getSupportLevel (spark/src/main/scala/org/apache/comet/serde/strings.scala:291-298) never inspects child types and lowers every non-foldable call to DataFusion's concat_ws, which accepts only Utf8 / LargeUtf8 / Utf8View and fails with Input was List(...) which is not a supported datatype for concat_ws function (datafusion-functions 54.1 concat_ws.rs). The query fails at native execution where Spark returns a result. There is no Comet test with an array argument.

Steps to reproduce

CREATE TABLE t USING parquet AS SELECT * FROM VALUES (array('a','b'), 'c') AS t(arr, s);
SELECT concat_ws(',', arr, s) FROM t;
-- also: SELECT concat_ws(',', split(s, ' ')) FROM t

Spark: a,b,c. Comet: native error Input was List(Field { name: "item", data_type: Utf8, ... }) which is not a supported datatype for concat_ws function.

Expected behavior

a,b,c — either flatten array arguments natively like Spark, or fall back to Spark for them.

Proposed solution

Short term: in getSupportLevel, return Unsupported (with a reason) when any child's dataType is an ArrayType, so the expression falls back (or goes through the codegen dispatcher). Longer term: a Spark-compatible native concat_ws that flattens list arguments (skipping null elements like Spark). Add a test with an array argument.

Additional context

Only the all-foldable case is declined today (and Spark constant-folds that anyway). Related: #3339 (null-separator crash, fixed by #3542 with a null-literal branch but no argument-type gate).