feat: implement getEscrowBalance function

[yunus-dev-codecrafter]

closes #83

Quality Checks
✅ All 87 unit tests passing
✅ TypeScript compilation successful
✅ ESLint validation passed
✅ Type safety verified
Implements the escrow balance fetching feature for the escrow lifecycle epic.

Changes:

• Add getEscrowBalance() function to accept accountId and horizonUrl
• Validate accountId with isValidPublicKey()
• Fetch account from Horizon API
• Extract native (XLM) balance from balances array
• Return BalanceInfo: {accountId, balance, lastModifiedLedger}
• Throw EscrowNotFoundError on 404, ValidationError on invalid input
• Add comprehensive unit tests covering:
  • Funded account with native balance
  • Account with zero balance
  • Account not found (404) error
  • Invalid public key validation
  • Other Horizon errors
  • Network failures
• Export new function from SDK index

Labels: escrow, network, sdk
Size: XS (~2-4 hours)

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[amina69]

@yunus-dev-codecrafter read the contributing .md before starting please, you are making pr to the wrong branch