Support schema-level `nullable: true` in OpenAPI 3.0

[halotukozak]

Description

OpenAPI 3.0 allows nullable: true on schema definitions independent of the required property list. Currently, nullability is inferred only from whether a property name is in the parent's required set. This means a property that is required but also nullable: true will be generated as non-nullable in Kotlin, which is incorrect.

Note: This topic needs additional research before implementation. The acceptance criteria below are preliminary and should be verified against the OpenAPI 3.0 spec and real-world usage patterns (e.g. how nullable interacts with allOf, oneOf, $ref, etc.). Update this issue with findings before starting work.

Acceptance criteria (preliminary — verify after research)

• SpecParser reads the nullable flag from schema definitions
• PropertyModel.nullable is true when either the property is not in required OR the schema has nullable: true
• TypeRef carries nullability information where relevant
• Generated data class properties have nullable Kotlin types when nullable: true is set on the schema
• Tests cover: required+nullable, required+non-nullable, optional+nullable, optional+non-nullable combinations