[REQ] [csharp] Handling of nullable and required properties

Is your feature request related to a problem? Please describe.

With the current csharp generators (except generichost), there is a confusion between required and nullable fields. Nullable fields and non-required fields are both using the csharp nullable operator "?". It leads to several problems.

• When a field is defined as not required and nullable, it's impossible to know if its value is missing or null.
• Model validation is sometimes incorrect (https://github.com/OpenAPITools/openapi-generator/issues/21090)

Describe the solution you'd like

Make a distinction between required and nullable fields. The solution used by generichost, using an Option struct for required fields seems good.

Describe alternatives you've considered

An alternative is moving to generichost but it's not possible for my project.

Start by comparing the existing C# generators with the generichost generator's handling of nullable and required properties. Read issue #21090 for the validation failure context and determine the affected generator models and tests. Done means required and nullable states are distinguishable without regressing model validation across the non-generichost C# generators.