[SPARK-51624][SQL] Propagate GetStructField metadata in CreateNamedStruct.dataType

### What changes were proposed in this pull request?

This change intends to propagate GetStructField metadata in CreateNamedStruct.dataType.

### Why are the changes needed?

This is important because dataType comparisons are important for optimizer rules such as SimplifyCasts, which can cascade down to more expression optimizations.

### Does this PR introduce _any_ user-facing change?

No.

### How was this patch tested?

Added unit test.

### Was this patch authored or co-authored using generative AI tooling?

No.

Closes #50412 from andylam-db/simplify-casts-comp.

Lead-authored-by: Andy Lam <andy.lam@databricks.com>
Co-authored-by: Wenchen Fan <cloud0fan@gmail.com>
Signed-off-by: Wenchen Fan <wenchen@databricks.com>

Author: andylam-db

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/complexTypeCreator.scala

@@ -457,6 +457,7 @@ case class CreateNamedStruct(children: Seq[Expression]) extends Expression with
       case (name, expr) =>
         val metadata = expr match {
           case ne: NamedExpression => ne.metadata
+          case gsf: GetStructField => gsf.metadata
           case _ => Metadata.empty
         }
         StructField(name.toString, expr.dataType, expr.nullable, metadata)

sql/core/src/test/scala/org/apache/spark/sql/ComplexTypesSuite.scala

@@ -19,10 +19,10 @@ package org.apache.spark.sql
 
 import scala.jdk.CollectionConverters._
 
-import org.apache.spark.sql.catalyst.expressions.CreateNamedStruct
+import org.apache.spark.sql.catalyst.expressions.{AttributeReference, CreateNamedStruct, GetStructField, Literal}
 import org.apache.spark.sql.catalyst.plans.logical.LogicalPlan
 import org.apache.spark.sql.test.SharedSparkSession
-import org.apache.spark.sql.types.{ArrayType, IntegerType, StructField, StructType}
+import org.apache.spark.sql.types.{ArrayType, IntegerType, MetadataBuilder, StringType, StructField, StructType}
 
 class ComplexTypesSuite extends QueryTest with SharedSparkSession {
   import testImplicits._
@@ -171,4 +171,18 @@ class ComplexTypesSuite extends QueryTest with SharedSparkSession {
     assert(df.schema == expectedSchema)
     checkAnswer(df, Seq(Row(Row(1, 2))))
   }
+
+  test("SPARK-51624: Propagate StructField metadata in CreateNamedStruct.dataType") {
+    val metadata = new MetadataBuilder().putString("comment", "hello").build()
+    val structRef = AttributeReference("s1",
+      StructType(StructField("col1", StringType, false, metadata) :: Nil))()
+    val createNamedStruct = CreateNamedStruct(
+      Seq(
+        Literal("a"),
+        GetStructField(structRef, 0)
+      )
+    )
+    val dataType = createNamedStruct.dataType
+    assert(dataType.asInstanceOf[StructType].fields.head.metadata == metadata)
+  }
 }