[VL] Local files node variant fallback fix

[felipepessoto]

What changes are proposed in this pull request?

Reading the Change Data Feed (CDF) of a Delta table that contains a VARIANT column
fails under the Velox native engine when the variant column is not projected:

spark.sql("""CREATE TABLE tbl (id INT, data VARIANT) USING delta
             TBLPROPERTIES ('delta.enableChangeDataFeed' = 'true')""")
spark.sql("""INSERT INTO tbl
             SELECT 1, parse_json('{"status":"active"}') UNION ALL
             SELECT 2, parse_json('{"status":"active"}')""")

// Works (variant projected → existing variant validation falls back to vanilla Spark)
spark.read.format("delta").option("readChangeFeed", "true").option("startingVersion", 0)
  .table("tbl").select("id", "data", "_change_type").show()

// Fails with GlutenNotSupportException: Type VariantType not supported
spark.read.format("delta").option("readChangeFeed", "true").option("startingVersion", 0)
  .table("tbl").select("id", "_change_type").show()

org.apache.gluten.exception.GlutenNotSupportException: Type VariantType not supported.
   at org.apache.gluten.expression.ConverterUtils$.getTypeNode(ConverterUtils.scala:248)
   at org.apache.gluten.substrait.rel.LocalFilesNode.buildNamedStruct(LocalFilesNode.java:115)
   at org.apache.gluten.substrait.rel.LocalFilesNode.toProtobuf(LocalFilesNode.java:199)
   at org.apache.gluten.backendsapi.velox.VeloxIteratorApi
        .$anonfun$genFirstStageIterator$3(VeloxIteratorApi.scala:224)
   ...

How was this patch tested?

New unit tests. Also tested with Delta unit tests.

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