Traffic goes through Istio (not Envoy Gateway), so cookie-based
consistent hashing must be on the Istio DestinationRule. Removes
the unused Envoy Gateway BackendTrafficPolicy.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
Fixes OIDC nonce validation failures with 2 replicas by using a
BackendTrafficPolicy with cookie-based consistent hashing instead of
Service-level ClientIP affinity (which doesn't work behind Envoy proxy).
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>