#24735 Madhus1218 fix csharp generichost oneof serialization

PR checklist

• Read the contribution guidelines.
• Run the following to build the project and update samples:

  ./mvnw clean package || exit
  ./bin/generate-samples.sh ./bin/configs/*.yaml || exit
  ./bin/utils/export_docs_generators.sh || exit
  

  (For Windows users, please run the script in WSL)
  Commit all changed files.
  This is important, as CI jobs will verify all generator outputs of your HEAD commit as it would merge with master.
  These must match the expectations made by your contribution.
  You may regenerate an individual generator by passing the relevant config(s) as an argument to the script, for example ./bin/generate-samples.sh bin/configs/java*.
  IMPORTANT: Do NOT purge/delete any folders/files (e.g. tests) when regenerating the samples as manually written tests may be removed.
• If your PR is targeting a particular programming language, @mention the technical committee members, so they are more likely to review the pull request.

---

Summary by cubic

Fixes C# generichost oneOf serialization when no discriminator. Old behavior wrapped everything in an object; new behavior serializes primitives/arrays as raw values and writes referenced model properties inline. This produces correct JSON; discriminator-based serialization is unchanged.

• Review Focus: modules/openapi-generator/src/main/resources/csharp/libraries/generichost/JsonConverter.mustache adds early returns for primitive/array and other non-object oneOf cases, and writes the selected referenced-model properties into the object before common properties. Samples were regenerated across all target frameworks to reflect the corrected output.
• Migration: Regenerate C# generichost clients. Update consumers/tests that depended on object-wrapped JSON for oneOf primitives, arrays/lists, or referenced models.