Resolves the identity links used to filter the assignee candidate list of a running process instance.
Parameters:
processInstanceId - the id of the process instance to resolve identity links for.
processDefinitionId - the id of the process definition; used to read the candidates-type extension element.
Returns:
the identity links to restrict the candidate list to, or an empty collection when no
candidates-type filter is configured, the configured value is unrecognized,
or the filter resolves to no users/groups. Callers treat an empty collection as
"no filter applied" (all users eligible), mirroring the task candidate behaviour.