feat: map Horizon error codes to HTTP status codes

[AlphaMaleBaDI]

Closes #222

Summary

Adds a centralized Horizon error code to HTTP status mapper and integrates it into the API error handling pipeline.

Changes

• Added src/utils/horizonStatusMapper.js
• Mapped common Horizon transaction and operation result codes to semantically appropriate HTTP status codes
• Integrated mapper into src/middleware/errorHandler.js
• Added dedicated unit tests for the mapper utility
• Added middleware tests covering Horizon error handling and fallback behavior

Supported Mappings

Horizon Code	HTTP Status
tx_bad_seq	409
tx_insufficient_fee	422
tx_bad_auth	403
op_no_trust	422
op_line_full	422
op_no_destination	404
op_underfunded	422
op_low_reserve	422
transaction_failed	422
not_found	404

Verification

PASS tests/errorHandler.test.js
PASS tests/horizonStatusMapper.test.js

Test Suites: 2 passed, 2 total
Tests: 11 passed, 11 total

The implementation preserves backward compatibility by falling back to the original Horizon response status when no mapped result code is available.

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