refactor(network): add network client for standardized requests

[Lakes41]

Summary

Implements a typed network client abstraction for Horizon and future Soroban RPC usage, ensuring consistent timeout handling, response parsing, and SDK error conversion for all network operations.

Changes

• Added NetworkClient class (src/network/index.ts#L81-L187): Typed HTTP client with get and post methods, built on top of existing fetchWithTimeout with consistent error handling and JSON parsing
• Added executeHorizonOperation helper (src/network/index.ts#L192-L213): Wraps Horizon server operations with timeout and consistent error handling
• Added executeSorobanOperation helper (src/network/index.ts#L218-L231): Wraps Soroban RPC operations with timeout and consistent error handling
• Updated fundTestnetAccount (src/wallet/index.ts#L198-L254): Refactored to use NetworkClient instead of direct fetchWithTimeout calls

Why

• Reduces duplication of request logic across SDK modules
• Ensures consistent timeout and error handling behavior
• Provides a shared abstraction for future Soroban RPC usage
• Converts network errors to consistent SDK error types

Testing

• All existing tests pass (npm run test)
• TypeScript compilation passes (npm run build)
• No breaking changes to public SDK behavior

Closes #157