Squashed 'ocaml-openapi/' changes from 4287b1c8..3799e796
3799e796 Add default values, validation, and field-level unions to OpenAPI generator
1841cd44 Complete OpenAPI generator with typed responses and nullable fields
7fdcbda9 Fix OpenAPI codegen to resolve $ref path parameters
git-subtree-dir: ocaml-openapi
git-subtree-split: 3799e79679cf44a553938a718259ef94f025748b