`jsonSchemaDialect` missing property and schema impacts

[cboitel]

As per https://spec.openapis.org/oas/v3.1.1.html#fixed-fields, there is an optional property named jsonSchemaDialect.

On this one (and maybe others), one should be aware of the followings:

1. it is optional and has a default
   • should the library return the default value if not provided ?
   • i think both should be possible (e.g. get vs get_or_default)
2. $schema property in schema definitions is expected to match the jsonSchemaDialect unless a specific$schema is provided:
   • again, should the library return the default value if not provided ?
   • moreover, dialect/schema used impacts supported properties in schema struct: should the library report invalid schema definitions against the specified json schema definition used ?