[csharp] Fixes incorrect property name

[devhl-labs]

Fixes #17552

There is a difference between Newtonsoft and System.Text.Json in that STJ requires the property name and the related constructor parameter name to be the same except for casing. In the event that the constructor parameter name is a keyword, we can use the @ symbol to escape it. This prevents us from having to rename both the parameter and the property just because one is a keyword.

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