[BUG] [CSHARP] [GENERICHOST] OptionProperty.mustache and JsonConverter.mustache use different `Option<T>` nullability handlings.

[laazik]

openapi-generator/modules/openapi-generator/src/main/resources/csharp/libraries/generichost/JsonConverter.mustache

Line 224 in d11d0ef

{{#lambda.camelcase_sanitize_param}}{{name}}{{/lambda.camelcase_sanitize_param}} = {{>OptionProperty}}JsonSerializer.Deserialize<DateTime{{#useDateTimeOffset}}Offset{{/useDateTimeOffset}}{{#isNullable}}?{{/isNullable}}>(ref utf8JsonReader, jsonSerializerOptions));

OptionProperty.mustache uses x-is-value-type to decide if ? is needed.

JsonConverter.mustache uses isNullable from the OpenAPI spec for the same decision on the same property.

Consequence is that marking a DateTime field nullable: true in spec has zero effect on the deserialization path — the non-nullable converter is used regardless.

Proposed fix is to mark all value types as nullable, which would match the contract set in the OptionProperty.mustache (which sets the value types as nullable).