Dependent dropdown validation for Workfront custom form metadata

Native dependent dropdowns in custom forms, where an admin defines which child options are available for each parent value, and the child dropdown shows only those options at runtime. It should work with forms used on projects, tasks, requests and documents, so the values map cleanly to AEM metadata.

Problem Statement
In Workfront custom forms, a dropdown can't limit its choices based on the value picked in another dropdown. Form logic can show or hide fields, but it can't filter the options inside a dropdown. Validation logic can block an invalid combination, but users still see every option and only find out after they choose wrong.

Example
A manufacturer's form has two fields: Model Range and Model Derivative. When the user picks Model Range = Range A, Model Derivative should only offer the derivatives that belong to Range A. Today it shows all of them, and nothing stops the user picking an invalid pair at runtime. 

Impact

• Users can submit invalid combinations, such as a derivative that doesn't belong to the chosen range.
• When assets are sent from Workfront to AEM Assets, AEM's metadata validation rejects the invalid value and leaves the field blank. If the field is optional, the asset goes into approval with the value missing. If it is mandatory, the asset is flagged "Mandatory Data Missing". Either way, an admin has to find and fix it by hand.