GatewayClient class abstract

Provider-neutral interface the SDK calls when it has an encrypted payload to hand off to the merchant backend.

Default behaviour: do nothing. The SDK ships in GatewayMode.payloadOnly by default, in which case it never instantiates a GatewayClient at all - the encrypted payload reaches the POS through the unified event stream and the POS handles the backend call itself.

gatewayHandoff behaviour. When the POS opts into GatewayMode.gatewayHandoff, it supplies a GatewayClientFactory that builds one client per initialize call. The SDK then calls GatewayClient.send once per encrypted-payload event.

Implementations MUST:

• resolve their Future with a GatewayHandoffResponse on any backend outcome the gateway accepted (approve, decline, pending),
• throw a GatewayError for transport-level failures (network, timeout, parse). The SDK translates the thrown error into the appropriate gatewayHandoffFailed event.

Implementations MUST NOT:

• decrypt the encrypted payload (the SDK does not have the keys; neither should the POS-side client),
• retry on the cardholder's behalf (the SDK's own retry loop handles transient transport failures and is deliberately conservative),
• persist the request or response anywhere - this layer is transport only.