HHH-9414: Convert MarkerObject to enum for serialization safety

[hoyeonyy]

Description

Fixes HHH-9414 by converting MarkerObject from class to enum, ensuring singleton identity is preserved across serialization/deserialization.

Problem:
Class-based MarkerObject creates new instances when deserialized, breaking == reference checks in session serialization scenarios (Spring Web Flow, clustered environments).

Solution:
Use enum to leverage Java's built-in serialization guarantees for singletons.

Changes

• Convert MarkerObject to enum with 6 constants
• Update 6 usage sites to reference enum values
• Add comprehensive serialization tests

Fixes https://hibernate.atlassian.net/browse/HHH-9414

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Please make sure that the following tasks are completed:
Tasks specific to HHH-9414 (Bug):

• Add test reproducing the bug
• Add entries as relevant to migration-guide.adoc OR check there are no breaking changes

[beikov]

I like the change, but the test seems useless to me. I'm not sure why the class was deprecated though as we use it internally. Can you chime in @gavinking?