Nullable reference types not reflected in generated properties

[YVbakker]

File: src/OpenApiSourceGenerator/Generators/PropertyGenerator.cs

Only the required OpenAPI keyword is mapped to the C# required modifier. Optional (non-required) reference-type properties (e.g. string, object references) are still emitted as non-nullable:

public class Pet
{
    public string Name { get; set; } // optional in the spec, but not nullable in C#
}

If the consuming project has <Nullable>enable</Nullable>, this produces CS8618 warnings for every optional reference-type property.

Suggested fix: Emit string? (or the appropriate nullable annotation) for non-required reference-type properties, or otherwise ensure generated code doesn't trigger nullable-context warnings by default.