Retrieve all custom fields scoped to a specific contract type. These are fields that are specific to a contract type (such as "MLS number" for a Listing contract type), distinct from team-level custom fields that apply to people or companies.

Use Cases:

• Build dynamic forms for a contract type
• Map external form fields (e.g., Sisu) to custom fields for a specific contract type
• Discover available fields when creating or updating contracts of this type