Append Part Dropdown Field Option(s)

Appends one or more options to a Dropdown custom field on parts. Existing options are preserved. Returns 400 if any option name is empty or whitespace-only after trimming. Returns 409 if any incoming name duplicates an existing option or another name in the same request (comparison is case-insensitive and trimmed); the error message lists all conflicting names. Within-request duplicates use the casing from the first occurrence of each duplicated name in the request. To replace the entire list, use PUT instead.