Xml - ObjectWithAttributesGene - add unsafecopy override and tests

[suarezrominajulieta]

Summary:

ObjectWithAttributesGene models XML elements with attributes (e.g., ).
The attributeNames set determines which fields are rendered as XML attributes vs. child elements.

Previously, ObjectWithAttributesGene did not override unsafeCopyValueFrom, causing mutation
operations to silently lose XML attribute metadata and potentially produce malformed XML during fuzzing.

Changes:

• Changed attributeNames from val to var to allow reassignment in the override.
• Added unsafeCopyValueFrom override: copies attributeNames when the source is an
  ObjectWithAttributesGene, or resets it to emptySet() when copying from a plain ObjectGene.

Tests:

Three new unit tests in ObjectWithAttributesGeneTest:

• Verifies attributeNames is correctly transferred between two ObjectWithAttributesGene instances.
• Verifies attributeNames is cleared when copying from a plain ObjectGene.
• Verifies equality is preserved when both genes share the same attributeNames.