test(wangapayfast-rs): add sandbox integration and contract tests

[mitch1009]

Summary

Increase confidence with integration and contract tests for API and ITN flows.

Scope

• Add opt-in sandbox integration test suite (env-gated)
• Add contract fixtures for expected response payloads
• Separate fast unit tests and slower integration tests

Acceptance Criteria

• Integration tests run when required env vars are set
• Contract fixtures validate response shape compatibility
• CI strategy documents when these tests execute