fix: update content type display_field to be optional and handle empty values

[demeyerthom]

This pull request addresses a bug where setting contenttype.display_field to empty caused issues during the apply phase. The changes ensure that display_field is now optional across the codebase and properly handles empty values. The key updates include modifications to the SDK models, resource logic, and documentation.

Bug Fixes and Functional Updates:

• .changes/unreleased/Fixed-20250801-153756.yaml: Added a changelog entry to document the fix for the contenttype.display_field issue.
• internal/resources/contenttype/model.go: Updated the Create, Update, Import, and Equal methods to use ValueStringPointer() for DisplayField, ensuring proper handling of nil values. [1] [2] [3] [4]

SDK Model Adjustments:

• internal/sdk/main.gen.go: Changed the DisplayField field in ContentType, ContentTypeCreate, and ContentTypeUpdate structs from string to *string to make it optional. [1] [2] [3]

OpenAPI Specification Updates:

• openapi.yaml: Removed displayField from the list of required fields for ContentType, ContentTypeCreate, and ContentTypeUpdate components. [1] [2] [3]

Documentation Updates:

• docs/resources/contenttype.md: Moved display_field from the "Required" to the "Optional" section to reflect its new status. [1] [2]