Service interfaces for testability

[rammrain]

Summary

Define interfaces for all core SDK services so consumers can mock the SDK in their tests.

Requirements

• OrderService interface — create order, get order status
• PaymentMethodService interface — get available payment methods
• TokenService interface — generate token, validate token
• Default implementations backed by the HTTP client
• Consumers can provide their own implementations or mock these in tests

Refs

• initial-docs/project1.md — interfaces for core services, testability
• initial-docs/project2.md — consumer-supplied HTTP client

Testing

Unit tests for default implementations (via stubbed HTTP). Verify interfaces are mockable with standard tools (Mockito). Target near-perfect coverage.