fix: reject xero-node API failures as Error objects

[miachillgood]

Summary

• reject SDK promise failures as Error objects instead of JSON strings
• keep the structured Xero response attached on the error object for downstream inspection
• add regression coverage for the new ApiError.toError() path

Why

Issues #500 and #741 both point at the same behavior: API failures are surfaced as non-Error rejections, and in newer paths they can even become plain strings via JSON.stringify(...). That breaks standard promise/error handling flows and strips away normal Error behavior like name, message, and stack handling.

This change keeps the structured response payload, but wraps it in a real Error instance so callers can handle failures predictably.

Validation

• npm test -- --runInBand
• npm run build

Closes #741