fix: convert nullable and keep examples for OpenAPI 3.1+

[Tony133]

Proposal:

Fixes #954

OpenAPI 3.1 uses JSON Schema 2020-12, where nullable does not exist and examples is an array keyword. When generating a 3.1+ document, the plugin passed nullable: true through unchanged (so nullability was silently lost by 3.1 readers) and downgraded examples to example, dropping every example after the first.

This is the symmetric conversion of #889, which handles the opposite direction (type: ['string', 'null'] → nullable: true for 3.0).

Changes (OpenAPI 3.1+ only):

• nullable: true is converted to type: [..., 'null']. As in OpenAPI 3.0, it only applies to an explicit type; without one it is dropped (Ajv refuses to compile such schemas anyway).
• enum is deliberately left untouched: Ajv only accepts null if enum lists it explicitly, and so does JSON Schema. Adding null to the enum would describe a value the validator rejects.
• examples arrays are kept in nested Schema Objects instead of being downgraded to example. The top-level media schema still moves examples to the Media Type Object, which is valid in 3.1 and is what Swagger UI displays.
• Nested schemas are now traversed when type is an array (e.g. ['object', 'null']); before, the examples resolution stopped at such schemas.

OpenAPI 3.0 output is unchanged.

Note:

convertJsonSchemaToOpenapi3 recurses into every value, including data such as default and x-* objects. A nullable key inside those would be touched, exactly as type already is on the 3.0 branch. This is a pre-existing limitation of the converter and should be handled separately.

[gurgunday]

LGTM, let's wait for one more review though