Pre-validates whether requested feature values would exceed AAS (Aruba Activation Server) license or system limits. For additive features (overlay, region, vrf, secondaryAccount), the requested value is added to current usage before checking the limit. For non-additive features (topologyMesh, qos, appExpress), the requested value is checked directly against the limit. Feature keys are automatically suffixed with 'Limit' internally (e.g., 'overlay' → 'overlayLimit'). Returns empty message if within limit, or descriptive error if exceeded.