Fix the cross-repository xDS hostname compatibility failure between Istio and grpc-java.

After upgrading an Istio-based proxyless service mesh, grpc-java can fail while processing route configuration for an otherwise valid Kubernetes service hostname. The failure is reported as `java.lang.IllegalArgumentException: Invalid pattern/domain name`. It occurs when Istio represents a virtual-host domain as an absolute fully qualified domain name, while the equivalent non-absolute form is used as the request hostname.

Reproduction: use Istio to generate the virtual-host domains for a Kubernetes service such as `echo.default.svc.cluster.local`, pass the generated absolute domain to grpc-java as an xDS virtual-host pattern, and attempt to select that virtual host for the equivalent hostname.

Expected behavior: Istio should be able to emit standards-compliant absolute DNS names, and grpc-java should accept and match the equivalent absolute and non-absolute forms without throwing. Existing exact and wildcard hostname matching, as well as unrelated Istio routing behavior, must not regress. Inspect both repositories and ensure their relevant tests pass.