C#: add opt-in nullable reference type output

[foobra]

Closes #3097.

What changed

• Add the opt-in --nullable-reference-types C# renderer option.
• Keep the existing output unchanged when the option is omitted.
• Emit nullable reference annotations for optional properties.
• Enable nullable-reference compiler directives and warning suppression when the option is enabled.
• For Newtonsoft.Json, avoid Required.DisallowNull on optional properties in this mode.
• Make the generated Newtonsoft FromJson result nullable in this mode.
• Add unit, type-level, and C# fixture coverage.

The option is intended for projects compiling with C# 8 or later. It can be combined with the existing --csharp-version 5/6 setting when consumers need the older generated syntax plus nullable reference annotations.

Validation

• npm run build
• Targeted Vitest tests: 13 passed
• Biome check: passed
• The C# fixture suite was attempted but could not run locally because this environment has .NET 6 and .NET 10 installed while the fixture targets .NET 8.

[github-actions]

Generated-output differences

8 files differ — 0 modified, 8 new, 0 deleted
23642 changed lines — +23642 / −0

Open the generated-output report →