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

[gaoyajun02]

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

[gaoyajun02]

cc @Ngone51 @mridulm @LuciferYang PTAL