test: cover Stellar DEX price source

[ProtonsAndElectrons]

Summary

• Add mocked Horizon coverage for the Stellar DEX source orderbook scenarios from Add Stellar DEX source integration tests with mocked Horizon #33.
• Fix the source to use Horizon.Server/Asset from stellar-sdk v11 instead of the missing top-level Server export.
• Derive prices from the midpoint of best bid/ask, fall back to one-sided books, return null for empty books, and surface Horizon failures to the oracle layer.
• Ensure native XLM uses the XLM/USDC orderbook and issued assets convert asset/XLM through XLM/USD.

Fixes #33

Validation

• npm test -- --runInBand (54 passed)
• npx jest test/stellarDex.test.js --runInBand (9 passed)
• npx jest test/stellarDex.test.js --runInBand --coverage --collectCoverageFrom=src/services/sources/stellarDex.js (stellarDex.js: 95.74% statements, 92.85% branches, 100% functions, 100% lines)
• git diff --check