feat(csharp): implement visitMapDeclaration and fix optional map fields

[muhabdulkadir]

Closes #

CSharp Visitor — Map Declaration Support visitMapDeclaration was silently skipped. The visit() dispatch returned early for map declarations with no output. Map types produced nothing in the generated C# file.

Each Concerto map declaration now emits a named class extending Dictionary<K,V>:

public class NextOfKin : Dictionary<string, string> {}
public class EmployeeDirectory : Dictionary<string, Employee> {}
public class EmployeeTShirtSizes : Dictionary<string, TShirtSizeType> {}

Optional map fields never emitted ?
writeField's map path had no isOptional() check. A field declared optional like o CompanyProperties companyProperties optional always emitted a non-nullable type. Fixed to emit Dictionary<K,V>? when optional.

Author Checklist

• Ensure you provide a DCO sign-off for your commits using the --signoff option of git commit.
• Vital features and changes captured in unit and/or integration tests
• Commits messages follow AP format
• Extend the documentation, if necessary
• Merging to main from fork:branchname