P4-3: Update Gateway interface with getPaymentType() and getGatewayId()

[tcheeric]

Description:
Extend Gateway interface for payment type and gateway identity.

Changes required:

• Add getPaymentType() and getGatewayId() to Gateway interface.
• Update interface documentation and usages.

Acceptance criteria:

• Gateway interface exposes payment type and gateway ID.
• Build fails if implementations are not updated.

Estimated hours: TBD
Priority: Medium
Branch name: refactor/phase-4-task-3
Dependencies: P4-1