Add numDeltaFiles/totalDeltaFilesByteSize to SnapshotDescriptor

Adds numDeltaFiles and totalDeltaFilesByteSize to the SnapshotDescriptor
trait with default implementations that throw UnsupportedOperationException,
and overrides them in Snapshot to delegate to logSegment. This lets callers
read these file statistics through the SnapshotDescriptor abstraction instead
of reaching into the snapshot's log segment directly.

Author: huan233usc

spark/src/main/scala/org/apache/spark/sql/delta/Snapshot.scala

@@ -67,6 +67,12 @@ trait SnapshotDescriptor extends DeltaLoggingProvider {
 
   def schema: StructType = metadata.schema
 
+  def numDeltaFiles: Long =
+    throw new UnsupportedOperationException("numDeltaFiles is not implemented for this descriptor")
+  def totalDeltaFilesByteSize: Long =
+    throw new UnsupportedOperationException(
+      "totalDeltaFilesByteSize is not implemented for this descriptor")
+
   def dataPath: Path
 
   protected[delta] def numOfFilesIfKnown: Option[Long]
@@ -124,6 +130,8 @@ class Snapshot(
   import org.apache.spark.sql.delta.implicits._
 
   override def dataPath: Path = deltaLog.dataPath
+  override def numDeltaFiles: Long = logSegment.numDeltaFiles
+  override def totalDeltaFilesByteSize: Long = logSegment.totalDeltaFilesByteSize
 
   protected def spark = SparkSession.active