[SPARK-36389][CORE][SHUFFLE] Revert the change that accepts negative …

…mapId in ShuffleBlockId

### What changes were proposed in this pull request?
With SPARK-32922, we added a change that ShuffleBlockId can have a negative mapId. This was to support push-based shuffle where -1 as mapId indicated a push-merged block. However with SPARK-32923, a different type of BlockId was introduced - ShuffleMergedId, but reverting the change to ShuffleBlockId was missed.

### Why are the changes needed?
This reverts the changes to `ShuffleBlockId` which will never have a negative mapId.

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

### How was this patch tested?
Modified the unit test to verify the newly added ShuffleMergedBlockId.

Closes apache#33616 from otterc/SPARK-36389.

Authored-by: Chandni Singh <singh.chandni@gmail.com>
Signed-off-by: Dongjoon Hyun <dongjoon@apache.org>

core/src/main/scala/org/apache/spark/storage/BlockId.scala

@@ -191,7 +191,7 @@ class UnrecognizedBlockId(name: String)
 @DeveloperApi
 object BlockId {
   val RDD = "rdd_([0-9]+)_([0-9]+)".r
-  val SHUFFLE = "shuffle_([0-9]+)_(-?[0-9]+)_([0-9]+)".r
+  val SHUFFLE = "shuffle_([0-9]+)_([0-9]+)_([0-9]+)".r
   val SHUFFLE_BATCH = "shuffle_([0-9]+)_([0-9]+)_([0-9]+)_([0-9]+)".r
   val SHUFFLE_DATA = "shuffle_([0-9]+)_([0-9]+)_([0-9]+).data".r
   val SHUFFLE_INDEX = "shuffle_([0-9]+)_([0-9]+)_([0-9]+).index".r

core/src/test/scala/org/apache/spark/storage/BlockIdSuite.scala

@@ -227,13 +227,13 @@ class BlockIdSuite extends SparkFunSuite {
   }
 
   test("merged shuffle id") {
-    val id = ShuffleBlockId(1, -1, 0)
-    assertSame(id, ShuffleBlockId(1, -1, 0))
-    assertDifferent(id, ShuffleBlockId(1, 1, 1))
-    assert(id.name === "shuffle_1_-1_0")
+    val id = ShuffleMergedBlockId(1, 2, 0)
+    assertSame(id, ShuffleMergedBlockId(1, 2, 0))
+    assertDifferent(id, ShuffleMergedBlockId(1, 3, 1))
+    assert(id.name === "shuffleMerged_1_2_0")
     assert(id.asRDDId === None)
     assert(id.shuffleId === 1)
-    assert(id.mapId === -1)
+    assert(id.shuffleMergeId === 2)
     assert(id.reduceId === 0)
     assertSame(id, BlockId(id.toString))
   }