[Bug] CAST(complex AS STRING) does not honour spark.sql.legacy.castComplexTypesToString.enabled

[andygrove]

Describe the bug

Spark's spark.sql.legacy.castComplexTypesToString.enabled configuration switches the format used by CAST(<struct|array|map> AS STRING):

• Default (false): structs render as {a, b, c}, arrays as [1, 2, 3], nulls as null.
• Legacy (true): structs render as [a, b, c], arrays as [1, 2, 3], nulls render as "" (empty string).

Comet's native struct/array-to-string formatter hard-codes the default-mode brackets and uses cast_options.null_string = "null". The legacy_cast_complex_to_string flag is not currently plumbed through the cast proto, so users who set spark.sql.legacy.castComplexTypesToString.enabled=true will see Comet output differ from Spark for CAST(<complex> AS STRING).

Surfaced by the cast audit (collection PR queue).

Steps to reproduce

SET spark.sql.legacy.castComplexTypesToString.enabled = true;
SELECT CAST(struct(1, 2, null) AS STRING);
-- Spark legacy: [1,2,]
-- Comet:        {1, 2, null}

Expected behavior

Either:

1. Plumb the conf through the Cast proto and switch the native formatter on legacy_cast_complex_to_string, OR
2. Downgrade (StructType|ArrayType|MapType, StringType) casts to Incompatible(Some(...)) when the conf is enabled.

Additional context

• Native impl: native/spark-expr/src/conversion_funcs/cast.rs (struct/array string formatters)
• Comet matrix: CometCast.canCastToString
• Spark conf: SQLConf.LEGACY_COMPLEX_TYPES_TO_STRING