[SPARK-50795][SQL][FOLLOWUP] Set isParsing to false for the timestamp…

… formatter in DESCRIBE AS JSON

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

This PR set isParsing to false for the timestamp formatter in DESCRIBE AS JSON, because the formatter is not used for parsing datetime strings

### Why are the changes needed?

Although it does not affect the final output due to the current fmt we use now being w/o 'S' portion, it can prevent potential bugs if we store/display higher-precision timestamps.

### Does this PR introduce _any_ user-facing change?
No

### How was this patch tested?

Existing tests

### Was this patch authored or co-authored using generative AI tooling?
No

Closes #50065 from yaooqinn/SPARK-50795.

Authored-by: Kent Yao <yao@apache.org>
Signed-off-by: Kent Yao <yao@apache.org>

sql/core/src/main/scala/org/apache/spark/sql/execution/command/DescribeRelationJsonCommand.scala

@@ -17,7 +17,7 @@
 
 package org.apache.spark.sql.execution.command
 
-import java.time.ZoneId
+import java.time.ZoneOffset
 
 import scala.collection.mutable
 
@@ -31,13 +31,7 @@ import org.apache.spark.sql.catalyst.catalog.{CatalogTable, CatalogTableType, Se
 import org.apache.spark.sql.catalyst.catalog.CatalogTypes.TablePartitionSpec
 import org.apache.spark.sql.catalyst.expressions.{Attribute, AttributeReference}
 import org.apache.spark.sql.catalyst.plans.logical.LogicalPlan
-import org.apache.spark.sql.catalyst.util.{
-  quoteIfNeeded,
-  DateFormatter,
-  DateTimeUtils,
-  Iso8601TimestampFormatter,
-  LegacyDateFormats
-}
+import org.apache.spark.sql.catalyst.util.{quoteIfNeeded, DateTimeUtils, TimestampFormatter}
 import org.apache.spark.sql.classic.ClassicConversions.castToImpl
 import org.apache.spark.sql.connector.catalog.CatalogV2Implicits._
 import org.apache.spark.sql.connector.catalog.V1Table
@@ -59,13 +53,8 @@ case class DescribeRelationJsonCommand(
         nullable = false,
         new MetadataBuilder().putString("comment", "JSON metadata of the table").build())()
     )) extends UnaryRunnableCommand {
-  private lazy val timestampFormatter = new Iso8601TimestampFormatter(
-    pattern = "yyyy-MM-dd'T'HH:mm:ss'Z'",
-    zoneId = ZoneId.of("UTC"),
-    locale = DateFormatter.defaultLocale,
-    legacyFormat = LegacyDateFormats.LENIENT_SIMPLE_DATE_FORMAT,
-    isParsing = true
-  )
+  private lazy val timestampFormatter =
+    TimestampFormatter("yyyy-MM-dd'T'HH:mm:ss'Z'", ZoneOffset.UTC, isParsing = false)
 
   override def run(sparkSession: SparkSession): Seq[Row] = {
     val jsonMap = mutable.LinkedHashMap[String, JValue]()