fix(csharp): harden decorator namespace mapping and required/scalar codegen semantics

[muhabdulkadir]

Closes #

This PR improves C# generation reliability by fixing decorator namespace resolution, centralizing required-property emission logic, and preventing scalar naming collisions in generated structs/converters.

Changes

• Ensures concerto.decorator references are mapped to the runtime metamodel namespace in generated C#.
• Adds centralized required-property decision logic for fields and relationships.
• Supports emitting required for non-optional reference properties when enabled.
• Keeps required opt-in via useRequiredForNonOptionalReferenceTypes.
• Extends map field generation to respect required emission when the flag is enabled.
• Adds reference-type detection helper used by required logic.
• Fixes CS0542 name collision when a scalar is named Value by generating RawValue member access in record structs and converter paths.

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