C# models could be records

The models generated by Kiota for the Csharp lang are currently `class`es. Making them `record`s would enable the usage of `with` among various benefits.

Would is be reasonable to do so?

The issue names no files, tests, or entry points. Start by locating the C# model-generation path and checking whether generated records preserve existing behavior; done would require an agreed scope and validated generated output, but the issue currently asks only whether the change is reasonable.

Written by the indexing model from the issue text.