Behavior: Generates a new, unpopulated content field model with default values. This does not persist the field; it only creates the template structure for the admin to configure and then attach to a content definition.

Authorization: Requires authentication and the user must be an admin user.

Side Effects: None. The field is not saved until the parent content definition is saved.

Error Scenarios: Returns 400 Bad Request if the user is not an admin or if the new field model could not be created.