#25549 fix(csharp): serialize object oneOf variants

Description

Fixes #25547

For C# generichost object oneOf models without a discriminator, write the active variant's properties alongside properties declared on the wrapper.

Tests

• Added a focused generator regression test and a .NET runtime regression test using the existing oneOf sample.
• Verified both variants in a minimal generated client round-trip; before the fix serialization returned {}.
• Regenerated C# generichost samples and built the .NET 10 OneOf and Petstore samples.
• Verified the full public Dropbox C# client builds and preserves file/folder metadata on serialization with this change and #25546's fix applied locally.

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Summary by cubic

Fixes #25547: C# generichost object oneOf models without a discriminator now serialize the active variant's properties alongside the wrapper's own properties, instead of producing {}.

• Updates JsonConverter.mustache to emit WriteProperties calls for non-primitive oneOf variants before wrapper properties.
• Adds a generator regression test, a .NET runtime regression test, and regenerated C# generichost samples.