#916 Fix ClassCastException when allOf array changes to type array

Fixes #887

When a property changes from allOf wrapping an array schema to a direct type: array, the left side stays a ComposedSchema after resolution while the right side is an ArraySchema. Casting both to ArraySchema threw ClassCastException.

Changes:

• copy items when merging composed schemas in addSchema
• use Schema#getItems() in ArraySchemaDiffResult instead of casting
• add regression test from the issue repro

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Summary by cubic

Fixes a crash when diffing array schemas that change from an allOf-wrapped array to a direct type: array. Prevents ClassCastException and ensures the specs compare equal.

• Bug Fixes
  • Copy items in addSchema when merging composed schemas.
  • Use Schema#getItems() in ArraySchemaDiffResult instead of casting.
  • Strengthen regression: assert no exception and that both specs are equal.