test: add comprehensive tests for orders advanced methods

[viktorcrypt]

Summary

Adds test coverage for three previously untested order methods: modify(), preview(), and closePosition().

Changes

• ✅ Added 11 new test cases in tests/orders-advanced.test.ts
• ✅ Covers all error scenarios: AuthenticationError, BadRequestError, NotFoundError, RateLimitError
• ✅ Follows existing test patterns (Jest + mocked fetch)
• ✅ All tests passing (142 total)

Testing

pnpm test -- orders-advanced.test.ts  # ✅ 11 passed
pnpm test                              # ✅ 142 passed

Motivation

These methods are used in production but had zero test coverage. This PR closes that gap.