Add support for nullable schemas (`nullable` and OAS 3.1 null unions)

[YVbakker]

Summary

Add support for nullable schemas via OpenAPI nullable and OpenAPI 3.1 union type arrays that include null.

Motivation

Nullability currently causes unsupported paths and potential NotImplementedException.

Scope

• Handle nullable: true in OpenAPI 3.0 style schemas.
• Handle OpenAPI 3.1 type arrays, e.g. type: ["string", "null"].
• Generate nullable CLR types where appropriate (T? for value types, nullable reference handling strategy for reference types).
• Keep behavior consistent with existing generator conventions.

Acceptance Criteria

• No NotImplementedException for nullable schema paths.
• Tests for value type and reference type nullability.
• Tests for OpenAPI 3.1 union-with-null cases.
• README notes nullability behavior and limitations.