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