C# oneOf wrappers: preserve payload data when discriminator is absent

[Copilot]

C# oneOf wrappers without a discriminator could deserialize into an IComposedTypeWrapper with all typed members unset, dropping object payload fields with no fallback access path. This change preserves that data by enabling AdditionalData on composed wrappers.

• Wrapper fallback for discriminator-less oneOf

  • Updated C# refiner flow to add serialization members with includeAdditionalProperties: true for model classes representing composed wrappers (OriginalComposedType != null).
  • This gives generated wrappers an AdditionalData bag and IAdditionalDataHolder, so unknown/unmapped payload fields remain accessible instead of being discarded.

• Targeted regression coverage

  • Extended C# refiner union-wrapper test to assert composed wrappers now:
    • contain an AdditionalData property
    • implement IAdditionalDataHolder

• Behavior impact

  • No discriminator/mapping logic was altered.
  • Change is scoped to C# composed wrapper models and only adds fallback data preservation.

// Before (discriminator-less object oneOf):
effect.Props.AddLoyaltyPointsEffectProps == null;
// payload fields were effectively lost

// After:
effect.Props.AddLoyaltyPointsEffectProps == null; // still possible
var raw = effect.Props.AdditionalData;            // payload preserved here

• Fixes C# oneOf deserialization silently loses data when no discriminator is defined #7573

[romanett]

Gavin Barron (@gavinbarron) I feel like this is finally a step in the right direction but for a library that still not supports oneOf for nullable reference types which get generated by Asp Net Core, this is a bold claim One of the goals of the project is to provide the best code generator support possible for OpenAPI and JSON Schema features.
Nearly one year after net10 release this is really not a great experience.
I am willing to spend some time to implement a solution to finally support that if you are okay to merge and give me some hints about the direction you want this to take shape.