New Errors and Testing guides

Errors now shows the responses the API actually sends. It covers every status code we saw in live testing, each error body, and what to do about it.

• 401 is plain text unauthorized, and a missing token returns 403.
• 429 returns {"message": "API rate limit exceeded", "request_id": "..."} with a Retry-After header.
• When a retried create returns 409, the first attempt went through.
• Log the request_id (on 429) or V1 uuid for support.

Testing your integration explains how to test without a sandbox. It covers which calls can email shoppers or leave data you can't delete, test data rules, and a go-live checklist.

Fixed: RateLimit-Reset is the number of seconds until the limit resets, not a Unix timestamp. Rate limiting is updated.