fix: support to_json on Spark 4.0

In Spark 4.0, StructsToJson is a RuntimeReplaceable whose replacement is
Invoke(Literal(StructsToJsonEvaluator), "evaluate", ...). Comet's serde
saw the post-replacement Invoke and fell back with "invoke is not
supported". Add a Spark 4-only matcher in CometExprShim that detects this
specific Invoke shape, reconstructs StructsToJson from the evaluator's
options, child, and timeZoneId, and recurses through exprToProtoInternal
so support-level checks still apply. Re-enable the four to_json tests
that were skipped on Spark 4.

Closes #3920

Author: andygrove

spark/src/main/spark-4.0/org/apache/comet/shims/CometExprShim.scala

@@ -20,7 +20,8 @@
 package org.apache.comet.shims
 
 import org.apache.spark.sql.catalyst.expressions._
-import org.apache.spark.sql.catalyst.expressions.objects.StaticInvoke
+import org.apache.spark.sql.catalyst.expressions.json.StructsToJsonEvaluator
+import org.apache.spark.sql.catalyst.expressions.objects.{Invoke, StaticInvoke}
 import org.apache.spark.sql.internal.SQLConf
 import org.apache.spark.sql.internal.types.StringTypeWithCollation
 import org.apache.spark.sql.types.{BinaryType, BooleanType, DataTypes, StringType}
@@ -114,6 +115,19 @@ trait CometExprShim extends CommonStringExprs {
       case k: KnownNotContainsNull =>
         exprToProtoInternal(k.child, inputs, binding)
 
+      // In Spark 4.0, StructsToJson is a RuntimeReplaceable whose replacement is
+      // Invoke(Literal(StructsToJsonEvaluator), "evaluate", ...). Reconstruct the
+      // original StructsToJson and recurse so support-level checks apply.
+      case i: Invoke =>
+        (i.targetObject, i.functionName, i.arguments) match {
+          case (Literal(evaluator: StructsToJsonEvaluator, _), "evaluate", Seq(child)) =>
+            exprToProtoInternal(
+              StructsToJson(evaluator.options, child, evaluator.timeZoneId),
+              inputs,
+              binding)
+          case _ => None
+        }
+
       case _ => None
     }
   }

spark/src/test/scala/org/apache/comet/CometExpressionSuite.scala

@@ -2271,8 +2271,6 @@ class CometExpressionSuite extends CometTestBase with AdaptiveSparkPlanHelper {
   }
 
   test("to_json") {
-    // TODO fix for Spark 4.0.0
-    assume(!isSpark40Plus)
     withSQLConf(CometConf.getExprAllowIncompatConfigKey(classOf[StructsToJson]) -> "true") {
       Seq(true, false).foreach { dictionaryEnabled =>
         withParquetTable(
@@ -2298,8 +2296,6 @@ class CometExpressionSuite extends CometTestBase with AdaptiveSparkPlanHelper {
   }
 
   test("to_json escaping of field names and string values") {
-    // TODO fix for Spark 4.0.0
-    assume(!isSpark40Plus)
     withSQLConf(CometConf.getExprAllowIncompatConfigKey(classOf[StructsToJson]) -> "true") {
       val gen = new DataGenerator(new Random(42))
       val chars = "\\'\"abc\t\r\n\f\b"
@@ -2329,8 +2325,6 @@ class CometExpressionSuite extends CometTestBase with AdaptiveSparkPlanHelper {
   }
 
   test("to_json unicode") {
-    // TODO fix for Spark 4.0.0
-    assume(!isSpark40Plus)
     withSQLConf(CometConf.getExprAllowIncompatConfigKey(classOf[StructsToJson]) -> "true") {
       Seq(true, false).foreach { dictionaryEnabled =>
         withParquetTable(

spark/src/test/scala/org/apache/comet/CometJsonExpressionSuite.scala

@@ -30,7 +30,6 @@ import org.apache.spark.sql.catalyst.expressions.{JsonToStructs, StructsToJson}
 import org.apache.spark.sql.execution.adaptive.AdaptiveSparkPlanHelper
 import org.apache.spark.sql.functions._
 
-import org.apache.comet.CometSparkSessionExtensions.isSpark40Plus
 import org.apache.comet.serde.CometStructsToJson
 import org.apache.comet.testing.{DataGenOptions, ParquetGenerator, SchemaGenOptions}
 
@@ -48,7 +47,6 @@ class CometJsonExpressionSuite extends CometTestBase with AdaptiveSparkPlanHelpe
   }
 
   test("to_json - all supported types") {
-    assume(!isSpark40Plus)
     withTempDir { dir =>
       val path = new Path(dir.toURI.toString, "test.parquet")
       val filename = path.toString