[SPARK-52923][CORE] Allow ShuffleManager to control push merge during shuffle registration

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

This PR moves the `shuffleManager.registerShuffle()` call to occur after the initialization of `_shuffleMergeAllowed` in `ShuffleDependency`.

### Why are the changes needed?

While `spark.shuffle.push.enabled` provides global control over push-based shuffle, there are scenarios requiring more granular control: 
- Mass spark application migration scenarios where different jobs may need different shuffle strategies
- Remote shuffle manager(e.g. celeborn/uniffle) need shuffle-level fallback capabilities to push-based shuffle
- Dynamic decision making based on shuffle characteristics during shuffle registration

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

No, this is an internal refactoring that maintains backward compatibility. The default behavior remains unchanged.

### How was this patch tested?

- Existing unit tests continue to pass
- The change only affects the order of initialization, not the logic

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

No

Closes #51629 from gaoyajun02/SPARK-52923.

Authored-by: gaoyajun02 <gaoyajun02@meituan.com>
Signed-off-by: Mridul Muralidharan <mridul<at>gmail.com>

Author: gaoyajun02

core/src/main/scala/org/apache/spark/Dependency.scala

@@ -127,15 +127,15 @@ class ShuffleDependency[K: ClassTag, V: ClassTag, C: ClassTag](
 
   val shuffleId: Int = _rdd.context.newShuffleId()
 
-  val shuffleHandle: ShuffleHandle = _rdd.context.env.shuffleManager.registerShuffle(
-    shuffleId, this)
-
   private[this] val numPartitions = rdd.partitions.length
 
   // By default, shuffle merge is allowed for ShuffleDependency if push based shuffle
   // is enabled
   private[this] var _shuffleMergeAllowed = canShuffleMergeBeEnabled()
 
+  val shuffleHandle: ShuffleHandle = _rdd.context.env.shuffleManager.registerShuffle(
+    shuffleId, this)
+
   private[spark] def setShuffleMergeAllowed(shuffleMergeAllowed: Boolean): Unit = {
     _shuffleMergeAllowed = shuffleMergeAllowed
   }