[DoRA] Add serialization and integration tests for DoRA

[buildwithsuhana]

Description

Ensures that DoRA-enabled layers are fully integrated into the Keras ecosystem, focusing on model persistence.

• Adds tests to saving_lib_test.py verifying that dora_magnitude, dora_kernel_a, and dora_kernel_b are correctly saved and loaded.
• Confirms that get_config/from_config correctly preserves DoRA state.
• Ensures compatibility with both high-level model.save() and low-level weight loading.

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[gemini-code-assist]

Code Review

This pull request adds a new unit test test_h5_io_store_dora to verify saving and loading DoRA weights using H5IOStore. However, the test calls enable_dora and accesses DoRA-related attributes on the Dense layer, which are not currently implemented and will cause an AttributeError. Additionally, the H5IOStore opened in read mode is not closed, which could lead to an unreleased file handle.