[CT-52] Write escrow edge-case tests: double-release, refund-after-release, unauthorized caller

[yusuftomilola]

Overview

Beyond the happy path, the funds-safety-critical edge cases need explicit coverage: releasing twice, refunding an already-released escrow, releasing/refunding an unknown escrow_id, and a caller without the required authorization from [CT-49].

Tasks

• Write a test for each of: double-release, refund-after-release, release-of-nonexistent-escrow, refund-of-nonexistent-escrow, unauthorized release, unauthorized refund.

Acceptance Criteria

• Every edge case above has a test asserting the call is rejected, not silently accepted.

Dependencies

Depends on [CT-49], [CT-51].

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.