New ca-east cache region available in staging and canary

A new ca-east geographic region on OVH bare metal is now available for Kura, gated to staging and canary behind TUIST_KURA_AVAILABLE_REGIONS while the OVH integration is validated. It replaces the throwaway ovh-staging region, giving canary a real geographic landing option. Production continues to serve us-east and us-west from OVH and is not affected by this rollout.