[SPARK-53581][CORE] Fix potential thread-safety issue for mapTaskIds.add()

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

This a followup for #47037. This PR wraps up the synchronize lock for invocation of `OpenHashSet.add()` in `IndexShuffleBlockResolver`.

### Why are the changes needed?

`OpenHashSet` is not thread safe. We should enfoce synchronize lock when invokes the add function to ensure the thread-safety.

### Does this PR introduce _any_ user-facing change?

No.

### How was this patch tested?

N/A.

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

No.

Closes #52337 from Ngone51/fix-thread-safety.

Authored-by: Yi Wu <[email protected]>
Signed-off-by: Dongjoon Hyun <[email protected]>

Author: Ngone51

core/src/main/scala/org/apache/spark/shuffle/IndexShuffleBlockResolver.scala

@@ -310,13 +310,13 @@ private[spark] class IndexShuffleBlockResolver(
             val mapTaskIds = taskIdMapsForShuffle.computeIfAbsent(
               shuffleId, _ => new OpenHashSet[Long](8)
             )
-            mapTaskIds.add(mapId)
+            mapTaskIds.synchronized { mapTaskIds.add(mapId) }
 
           case ShuffleDataBlockId(shuffleId, mapId, _) =>
             val mapTaskIds = taskIdMapsForShuffle.computeIfAbsent(
               shuffleId, _ => new OpenHashSet[Long](8)
             )
-            mapTaskIds.add(mapId)
+            mapTaskIds.synchronized { mapTaskIds.add(mapId) }
 
           case _ => // Unreachable
         }