Fix WP_Object_Cache deep copy for objects nested in arrays

[himanshupathak95]

Trac ticket: https://core.trac.wordpress.org/ticket/30430

Objects stored in arrays within WP_Object_Cache were sharing references with the original data, causing unexpected mutations when either the original or cached data was modified.

This change adds deep copying functionality to properly clone objects nested within arrays at any level, ensuring cached data remains isolated from the original data structures.

Changes:

• Add deep_copy() method for recursive object cloning in arrays
• Update set() method to deep copy arrays containing objects
• Update get() method to deep copy arrays when retrieving from cache
• Add unit tests covering shallow and deep nesting scenarios

Fixes reference sharing issue while maintaining performance for primitive arrays.

Follows the implementation of the patch - https://core.trac.wordpress.org/attachment/ticket/30430/wp-deep-copy-withclass2.diff

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