[SPARK-51401][SQL] Change ExplainUtils.generateFieldString to directly call QueryPlan.generateFieldString

[LuciferYang]

What changes were proposed in this pull request?

This pr change ExplainUtils.generateFieldString to directly call QueryPlan.generateFieldString because these are identical methods.

spark/sql/core/src/main/scala/org/apache/spark/sql/execution/ExplainUtils.scala

Lines 300 to 307 in 4b4bdcf

	// TODO(nemanja.petrovic@databricks.com) Delete method as it is duplicated in QueryPlan.scala.
	def generateFieldString(fieldName: String, values: Any): String = values match {
	case iter: Iterable[_] if (iter.size == 0) => s"${fieldName}: []"
	case iter: Iterable[_] => s"${fieldName} [${iter.size}]: ${iter.mkString("[", ", ", "]")}"
	case str: String if (str == null || str.isEmpty) => s"${fieldName}: None"
	case str: String => s"${fieldName}: ${str}"
	case _ => throw new IllegalArgumentException(s"Unsupported type for argument values: $values")
	}

spark/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/plans/QueryPlan.scala

Lines 789 to 795 in 4b4bdcf

	def generateFieldString(fieldName: String, values: Any): String = values match {
	case iter: Iterable[_] if (iter.size == 0) => s"${fieldName}: []"
	case iter: Iterable[_] => s"${fieldName} [${iter.size}]: ${iter.mkString("[", ", ", "]")}"
	case str: String if (str == null || str.isEmpty) => s"${fieldName}: None"
	case str: String => s"${fieldName}: ${str}"
	case _ => throw new IllegalArgumentException(s"Unsupported type for argument values: $values")
	}

Meanwhile, this is a TODO left by nemanjapetr-db in SPARK-50739.

Why are the changes needed?

Remove duplicated code.

Does this PR introduce any user-facing change?

No

How was this patch tested?

Pass Github Actions

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

No

[sumitsingh-in]

LGTM

[dongjoon-hyun]

+1, LGTM. Thank you, @LuciferYang .
Merged to master/4.0.