[maxx-GENIUS]

Summary

This PR adds comprehensive unit test coverage and utility helpers for Stellar memo byte limits, decimal rounding, and asset code validation in backend/src/utils/stellar.ts and backend/src/utils/__tests__/stellar.test.ts.

Closes #2089

Details of Changes

• Stellar Memo Validation (isValidStellarMemo / assertValidStellarMemo):
  • Enforces the 28-byte protocol boundary using UTF-8 byte counting (Buffer.byteLength).
  • Validates boundary cases (empty string, 28 bytes ASCII, multi-byte UTF-8 emojis exceeding 28 bytes) and non-string inputs.
• Asset Code Validation (isValidAssetCode / assertValidAssetCode):
  • Validates 1-12 alphanumeric characters (AlphaNum4, AlphaNum12, native, XLM).
  • Rejects asset codes exceeding 12 characters, containing spaces, special characters, or invalid types.
• Decimal Rounding & Amount Parsing (formatStellarAmount / parseStellarAmount):
  • Formats amounts with exact 7-decimal precision (stroops representation).
  • Applies standard half-up decimal rounding to eliminate floating-point drift.
  • Rejects negative, NaN, or non-finite inputs.

Verification & CI Proof

• npm run lint: Passed (0 errors)
• npm run build: Passed (0 errors)
• npm test -- src/utils/__tests__/stellar.test.ts: 16/16 tests passing (100% green)

[github-actions]

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