Fix APOD API error-response decoding and add tests

[spiegel-im-spiegel]

Summary

• add ErrAPODAPIResponse to represent structured APOD API error responses
• keep current io.TeeReader + bytes.Buffer decode structure and decode normal payload from buffered bytes
• classify APOD API error responses by:
  • status >= http.StatusBadRequest, or
  • status == 0 && code != ""
• avoid false APOD-error classification for non-error objects with empty code/status
• add focused tests for normal array/single decode and APOD error-response classification

Validation

• go test ./nasaapi/nasaapod ./nasaapi
• go test ./...