[csharp][generichost] Deserialize present-but-null nullable enums

[daberni]

For a property that is both required and nullable with an enum type, the generichost JsonConverter.Read(...) only assigned the backing Option when the raw JSON string was non-null. An explicit null therefore left the Option unset, and the required-property check then threw ArgumentException: Property is required ... — even though the property was present (just null).

Non-enum nullable properties (string, int, DateTime, …) already assign the Option unconditionally; only the enum branch had the null-skipping guard.

Reproduce

Deserialize a model with a required + nullable enum whose JSON value is null, e.g. RequiredClass with "required_nullable_enum_string": null:

System.ArgumentException : Property is required for class RequiredClass. (Parameter 'requiredNullableEnumString')

Expected: deserializes with the value null.

Fix

csharp/libraries/generichost/JsonConverter.mustache: for the string-enum read, assign the Option unconditionally when the property is nullable, mapping a null raw value to a null enum value. Non-nullable enums keep the existing guard, so their generated output is unchanged (scoped via {{#isNullable}}).

Test

Adds RequiredClassNullableEnumTests to the net8 NullReferenceTypes sample, deserializing RequiredClass with required_nullable_enum_string set to null. It throws before the fix and passes after (verified locally, red→green).

PR checklist

• Read the contribution guidelines.
• Pull Request title and description describe the change and how to validate it.
• Built the project and regenerated the affected samples (./bin/generate-samples.sh ./bin/configs/csharp-generichost-*.yaml); committed all changes. The change is scoped to the generichost C# template, so only generichost samples are affected.
• Filed against master.
• No existing issue (searched) — reproduction described above.
• Technical committee: @devhl-labs (C# GenericHost)

---

Summary by cubic

Fixes C# generichost deserialization for required nullable enums so a present-but-null value no longer triggers a “Property is required” error. Non-nullable enums are unchanged.

• Bug Fixes
  • Update csharp/libraries/generichost/JsonConverter.mustache to always set the backing Option for nullable enums, mapping a null raw value to a null enum.
  • Add generator test CSharpClientCodegenTest#testGenericHostNullableEnumDeserializesPresentNull to verify nullable enums accept null and non-nullable enums keep the guard.
  • Regenerate affected generichost samples.

^{Written for commit 1458996. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 38 files

_{Re-trigger cubic}

[wing328]

thanks for the PR

cc @devhl-labs