feat: human-readable Horizon error message translator

[FaithOnuh]

Summary

Resolves #202

Extends the error handler to translate cryptic Horizon result codes (e.g. tx_bad_seq, op_no_trust) into plain-English messages automatically.

Changes

• src/utils/horizonErrors.js (new): Translator with 17 common Horizon error codes mapped to plain-English descriptions covering transaction errors (tx_*) and operation errors (op_*).
• src/middleware/errorHandler.js: Updated HorizonError handler to extract result_codes from Horizon extras, translate to human-readable message, and include the original code field in the response.
• tests/api.test.js: Added 12 new tests verifying all covered codes, null returns for unknown codes, and integration through the error handler. Fixed a pre-existing extra closing bracket.

Error Response Shape

{
  "success": false,
  "error": {
    "type": "HorizonError",
    "title": "Transaction Failed",
    "detail": "...",
    "status": 400,
    "extras": { "result_codes": { "transaction": "tx_bad_seq" } },
    "code": "tx_bad_seq",
    "message": "Transaction sequence number does not match the account's current sequence. Reload the account and rebuild the transaction."
  }
}

Covered Error Codes (17)

tx_bad_seq, tx_insufficient_fee, tx_bad_auth, tx_no_source_account, tx_bad_auth_extra, tx_internal_error, tx_not_supported, tx_fee_bump_inner_failed, op_no_destination, op_no_trust, op_line_full, op_underfunded, op_low_reserve, op_bad_auth, op_no_account, op_not_authorized, op_malformed

Tests

85 passed, 2 skipped (pre-existing skips)

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[Sulex45]

resolve Conflict in test/api.test