feat(wangapayfast-rs): complete PayFast endpoint parity with tests

[mitch1009]

Summary

Audit and close endpoint coverage gaps so the SDK maps cleanly to supported PayFast operations.

Scope

• Compare current client coverage vs PayFast docs
• Implement missing endpoints with typed request/response models
• Add tests per endpoint contract

Acceptance Criteria

• Endpoint parity checklist added to repo
• Missing endpoints implemented or explicitly documented as unsupported
• Tests cover success + failure responses