[OCaml][Fix] Unreferenced enum + Direct recursive types

[sir4ur0n]

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Summary by cubic

Fixes crashes in the OCaml client generator when enums appear in anyOf/allOf/oneOf, and adds robust support for direct self-recursive models across properties, arrays, and maps. Adds targeted tests and samples, and updates CI to OCaml 5.3.

• Bug Fixes

  • Recursively collect enums in composed schemas (anyOf/allOf/oneOf), including direct branch enums and nested $ref; prefix composed enum types with Enums.
  • Make enum hashing order-independent with Set, fixing “unreferenced enum” and supporting empty-string enum values.

• New Features

  • Support direct recursive models by normalizing self-references to t across fields and container items; handle required recursive fields and multiple recursive properties.
  • Add 3.0 test specs and samples for composed-schema enums and recursion; update OCaml generator help text.

^{Written for commit f3e7e8d. Summary will update on new commits.}