Missing OpenAPI feature support

[YVbakker]

Files: src/OpenApiSourceGenerator/Generators/PropertyGenerator.cs, src/OpenApiSourceGenerator/Mappers/TypeMapper.cs

Not currently handled:

• enum schemas (should likely generate a C# enum)
• format (e.g. int64 → long, int32 → int, date-time → DateTimeOffset, uuid → Guid, byte → byte[])
• nullable keyword / OpenAPI 3.1 type arrays
• oneOf / anyOf / allOf
• additionalProperties (dictionary/map types)

These currently throw NotImplementedException when encountered.

Suggested fix: Track as a backlog of incremental feature support. At minimum, document these as known limitations in the README so consumers aren't surprised by a crash. Longer term, use format for more precise primitive type mapping and add enum generation.