Validate user availablity when assigning to workflow on PBS Creation/Edition

Feature Request: Workflow assignment validation for PBS creation

Summary:
Ensure that only users who are eligible for a specific workflow step appear as assignment options during PBS creation or editing. If a workflow assignment cannot be completed, the system should clearly warn the user instead of failing silently.

Problem Statement:
Currently, when assigning users during PBS creation or editing, the system shows all users assigned to the parent for every step, regardless of whether they are actually valid for that specific step. This creates confusion and leads users to make assignments that cannot be completed successfully. When an invalid user is assigned, the workflow assignment fails silently, leaving the person creating the PBS unaware that the assignment did not work. This lack of validation and feedback makes the process inefficient and increases the risk of setup errors.

Proposed Solution:
Update the PBS creation and editing workflow so that each step only displays users who are valid and available for assignment in that specific step.
Validate assignments in real time or at the point of submission to ensure that invalid selections cannot be applied without notice.
If an assignment fails for any reason, display a clear warning message explaining which assignment failed and why.
Ensure the warning appears before the PBS setup is finalized so the user can correct the issue immediately.
Preserve the current workflow structure while improving assignment accuracy and transparency.

Benefits:
Reduces user confusion by showing only valid assignment options for each workflow step.
Prevents silent assignment failures that can disrupt workflow setup and execution.
Improves efficiency by helping users complete PBS creation correctly the first time.
Increases trust in the system through clearer validation and error feedback.