v1.7.6: panic-safe error parsing on 4xx

• Make JSON error parsing panic-safe when bodies are empty/malformed.

• parseJSONError: fallback to status/body snippet; no panics.

• (*JSONError).Error(): safe when Err slice empty; falls back to http.StatusText.

• Add tests for 404 cases (well-formed, success-shaped, empty).

• No API changes.