Returns root-level union validation errors combined with
per-branch validation results.
The returned value has both NestedValidationResult properties
(errors, isValid, descriptor, seenValue) and tuple-indexed
branch results ([0], [1], etc.).
Validation result type returned by
UnionSchemaBuilder.validate(). ExtendsValidationResultwith:getNestedErrorsfor root-level union errors and per-branch validation results