fix(csharp): correct Double→double, field array/nullable order, and optional relationship type

[muhabdulkadir]

Closes #

• Double mapped to float instead of double; Concerto's 64-bit Double was mapped to C#'s 32-bit float. Fixed to double.

• writeField parameter order (array / nullableType swapped) toCSharpProperty takes (..., array, nullableType, ...). The call in writeField had them reversed, causing optional arrays to emit string?[] (array of nullable strings) instead of string[]? (optional array).

• visitRelationship never emitted ? for optional relationships. The isOptional() check existed for fields but was missing entirely from visitRelationship. Optional relationships always emitted a non-nullable type.

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