PASS: active Transport Driver user remains eligible without a physical driver_id
PASS: validate-assignment succeeds with application_user_id and no driver_id
PASS: standalone vehicle and schedule pre-validation succeeds before an assignee is selected
PASS: inactive or non-Driver application user is rejected
PASS: vehicle schedule conflict remains a blocking HTTP 409
PASS: legacy driver_id payload remains compatible when an explicit actor mapping exists

6 checks passed; 0 failed.