Add 404 status code to fallback conditions in hub_sync.go so the
integration gracefully falls back to GitHub mirror when primary
hub-data.crowdsec.net returns 404.
Add http.StatusNotFound to fetchIndexHTTPFromURL fallback
Add http.StatusNotFound to fetchWithLimitFromURL fallback
Update crowdsec_integration.sh to check hub availability
Skip hub preset tests gracefully when hub is unavailable
Fixes CI failure when CrowdSec Hub API is temporarily unavailable