[BE-122] Unit tests for the stellar module

[mftee]

Overview

backend/src/stellar/ has 2 files and no tests, and stellar-sdk is a dependency. This module bridges to an external network, which makes both its success and failure paths worth pinning down.

Tasks

• Mock the Stellar SDK and Horizon HTTP layer — tests must never hit a live network.
• Test transaction construction: correct memo, correct asset, correct destination.
• Test network failure, timeout, and malformed-response handling — each should produce a typed error, never an unhandled rejection.
• Test that a failed anchoring never reports success to callers.
• Test testnet vs mainnet configuration selection, and that mainnet is never used by default.
• Target ≥75% coverage.

Acceptance Criteria

• npm run test -- stellar passes at ≥75% coverage with no network access.
• Every external failure mode maps to a typed error.
• A test proves the default network is not mainnet.

Notes for Contributors

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