Reorganize aggregate schema tests into category-based structure

Remove _parked/ and issues/ directories. All tests are now organized
by what they test, not by upstream issue number.

New structure under components/:
  allof_with_properties/
    same_level/                       - properties at same level as allOf
    additional_properties/            - allOf members with additionalProperties
    additional_properties_merge/      - additionalProperties merge matrix
    format/                           - allOf to apply format constraints
  nested_aggregate/                   - combined spec with 6 scenarios:
    arrays of anyOf, objects with anyOf/oneOf, allOf+oneOf,
    oneOf with nested allOf field preservation, composition enums
  any_of_single_ref/                  - anyOf with single $ref
  any_of_enums/                       - enums inside anyOf members
  recursive_one_of/                   - circular oneOf references
  recursive_all_of/                   - recursive allOf self-reference
  one_of_string_enums/                - oneOf with enum variants
  one_of_discriminator_multi_mapping/ - discriminator with multiple mappings

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: deepmap-marcinr