Address review feedback on PR #101:
1. Capture resp.StatusCode and Retry-After header *before* passing resp
to handleResponse, making ownership transfer explicit. Previously the
caller read resp.StatusCode after handleResponse had closed the body —
correct but fragile coupling.
2. Add panic guard ensuring backoff slice length equals maxAttempts-1.
Previously the relationship was implicit and could silently break if
maxAttempts were changed without updating the default backoff.