[AURON #1730] Fix data size calculation in Celeborn shuffle writer (#1731)

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Closes #1730 

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Author: DDDominik

thirdparty/auron-celeborn-0.6/src/main/scala/org/apache/spark/sql/execution/auron/shuffle/celeborn/AuronCelebornShuffleWriter.scala

@@ -76,4 +76,24 @@ class AuronCelebornShuffleWriter[K, V](
       Shims.get
         .getMapStatus(blockManagerId, celebornPartitionWriter.getPartitionLengthMap, mapId))
   }
+
+  // Override stop to use partition length map directly instead of rssStop's mapStatus
+  // because celeborn writer doesn't populate partition sizes correctly when using native writer
+  override def stop(success: Boolean): Option[MapStatus] = {
+    if (!success) {
+      celebornShuffleWriter.stop(success)
+      return None
+    }
+
+    celebornShuffleWriter.write(Iterator.empty)
+    celebornShuffleWriter.stop(success)
+
+    // Always use getPartitionLengthMap for Celeborn to get correct partition sizes
+    val blockManagerId = SparkEnv.get.blockManager.shuffleServerId
+    Some(
+      Shims.get.getMapStatus(
+        blockManagerId,
+        celebornPartitionWriter.getPartitionLengthMap,
+        taskContext.partitionId()))
+  }
 }