[TT-16410] Validation fixes for OAS 3.1 from community

[pvormste]

Based on this community PR: chanceattoast#1

This pull request adds support for JSON Schema 2020-12 compliant validation when working with OpenAPI 3.1 documents. The changes ensure that validation logic and schema handling are updated to accommodate features and requirements specific to JSON Schema 2020-12, and the new validation mode is automatically enabled for OpenAPI 3.1 documents.

OpenAPI 3.1 / JSON Schema 2020-12 Support:

• Introduced the EnableJSONSchema2020Validation option in the validation options, allowing the validator to perform JSON Schema 2020-12 compliant validation for OpenAPI 3.1 documents (openapi3/validation_options.go, .github/docs/openapi3.txt). [1] [2]
• Updated the CLI (cmd/validate/main.go) to detect OpenAPI 3.1 documents and automatically enable JSON Schema 2020-12 validation when appropriate.

Schema Validation Logic:

• Modified schema validation to support the 'null' type only when JSON Schema 2020-12 validation is enabled, and to pass the relevant option to default value and example validation (openapi3/schema.go, openapi3/example_validation.go). [1] [2] [3]
• Adjusted handling of extra sibling fields in schema references: such fields are only allowed under JSON Schema 2020-12 validation, enforcing stricter validation otherwise (openapi3/refs.go, openapi3/refs.tmpl). [1] [2]

Other:

• Added a new field jsonSchema2020ValidationEnabled to ValidationOptions to track if the new validation mode is active (openapi3/validation_options.go).
• Minor: Updated the code generation comment in refs.tmpl for clarity.