[trivial] [bounty] Add transaction builder tests to @x402/wallet package #8

Overview

The @x402/wallet package provides Stellar transaction building utilities but has no unit tests. Add comprehensive tests.

Requirements

• Test building a payment transaction with correct amount, asset, destination
• Test fee calculation
• Test transaction signing flow
• Test Horizon submission error handling
• Use mock Horizon responses (don't hit real network)

Skills

TypeScript, Jest, Stellar SDK

Estimated Complexity

Trivial (~2 hours)