[CT-55] Add an integration test running the Soroban specs against the real deployed testnet contract

[yusuftomilola]

Overview

escrow-contract.client.spec.ts and its sibling specs (escrow-submission.processor.spec.ts, soroban-rail.adapter.spec.ts, etc.) currently only exercise mocked RPC responses. Once [CT-53]/[CT-54] land, add a test that runs against the actual deployed testnet contract at least once, so the mocks are proven faithful to the real thing.

Tasks

• Add an integration test (likely gated behind an env flag so it doesn't run on every CI job, given testnet dependency) exercising create/release/refund/get_status against the real testnet contract from the backend's real client.

Acceptance Criteria

• At least one test proves the backend's Soroban integration works against a real deployed contract, not only mocks.

Dependencies

Depends on [CT-54].

Notes for Contributors

Requires Rust + Soroban. This is new scaffolding work — see README.md's documented contracts/ architecture and backend/src/payments/soroban/ for what the backend already expects to call. Comment below to be assigned.