VSN-2882/feat(driver-planning): prioritize drivers assigned in preparation

Add PreparationPlanningServiceClient + new optional query param prioritizeAssignedForPreparationDate on /api/user/driver/all-drivers-planning. Mirrors the chauffeur-side flow but resolves "assigned" via vsn-preparation- service. Takes precedence over prioritizeAssignedForDate when both are sent; falls through to the unprioritized query when the assigned list is empty.