feat: implement buildPaymentOp() transaction helper — closes #112

[spiffamani]

Summary

Implements buildPaymentOp() helper for building XLM payment operations, closing #112.

Changes

• src/transactions/index.ts — Added buildPaymentOp()
• src/index.ts — Exported buildPaymentOp from public surface
• tests/unit/transactions/buildPaymentOp.test.ts — 9 unit tests

Usage

import { buildPaymentOp } from '@petad/stellar-sdk';

const op = buildPaymentOp({
  destination: 'GBBD...LA5',
  amount: '10.5',
});

Tasks Completed

• Accepts destination, amount, asset (default native XLM)
• Validates destination with isValidPublicKey()
• Validates amount with isValidAmount()
• Returns Stellar SDK Payment operation object
• Throws ValidationError for invalid inputs
• Unit tests: valid payment op, invalid destination throws, zero amount throws

Test Results

Tests: 87 passed, 87 total
Test Suites: 10 passed, 10 total

Checklist

• npm test passes
• No new runtime dependencies
• Narrowly scoped to described behavior

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[amina69]

@spiffamani you are making pr to the wrong branch, read the conributing.md