Full JSON Schema 2020-12 Alignment
OpenAPI 3.1 achieves 100% alignment with JSON Schema Draft 2020-12. This removes the historic superset/subset discrepancies of OpenAPI 3.0, allowing tools to use standard JSON Schema validators directly.
Nullability: The Departure of nullable: true
In OpenAPI 3.0, nullable values were declared with `nullable: true`. In OpenAPI 3.1, nullability uses standard JSON Schema type arrays: `type: ["string", "null"]` or `anyOf: [{type: "string"}, {type: "null"}]`.