fix: resolve undefined vars in HorizonError branch and add tests (#256)

[sanctus-mvp]

Summary

Fixes the undefined variable bug in the HorizonError branch of errorHandler.js (issue #2) and adds test coverage to prevent regressions.

Changes

Bug fix — src/middleware/errorHandler.js

• translateHorizonError was imported but never called, leaving code and humanMessage as undefined references that threw a ReferenceError at runtime
• Added const code = resultCode and const humanMessage = translateHorizonError(resultCode) to resolve both variables before they are spread into the response

Tests — tests/errorHandler.test.js

• 13 tests covering the full HorizonError branch
• Asserts success: false, error.type === "HorizonError", presence of title and detail
• Verifies mapped HTTP status codes (tx_bad_seq → 409, op_no_destination → 404)
• Explicit not.toThrow() assertion confirming no ReferenceError
• Covers translated message for recognised codes, absent message for unrecognised ones

Other

• Resolved a git merge conflict marker in package.json (node-cache version)

Test results

All 13 tests pass. Coverage for errorHandler.js: 90% statements / 89.65% lines / 100% functions — above the 80% threshold required by #256.

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Issue	Title
#256	Add tests for the HorizonError branch in errorHandler.js	Link to this issue

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