Support using xPolicies with routes that attach to ListenerSets

*Description*:
Currently (v1.8.1) attaching policies to routes that attach to `ListenerSet` instead of `Gateway` doesn't work. The policies get ignored - they don't have their status fields set and they don't affect anything (double-checked by dumping and verifying xDS).

Example that doesn't work:
```yaml
---
apiVersion: gateway.networking.k8s.io/v1
kind: Gateway
metadata:
namespace: envoy-gateway
name: gateway-1
spec:
gatewayClassName: envoy-gateway-class
listeners:
- name: http
protocol: HTTP
port: 80
allowedRoutes:
namespaces:
from: All
allowedListeners:
namespaces:
from: All
---
apiVersion: gateway.networking.k8s.io/v1
kind: ListenerSet
metadata:
namespace: default
name: listener-1
spec:
parentRef:
name: gateway-1
namespace: envoy-gateway
listeners:
- name: extra-http-one
protocol: HTTP
port: 8081
---
apiVersion: gateway.networking.k8s.io/v1
kind: HTTPRoute
metadata:
namespace: default
name: httproute-1
spec:
hostnames:
- gateway.envoyproxy.io
parentRefs:
- group: gateway.networking.k8s.io
kind: ListenerSet
name: listener-1
rules:
- matches:
- path:
value: "/http1"
backendRefs:
- name: service-1
port: 8080
---
apiVersion: gateway.envoyproxy.io/v1alpha1
kind: BackendTrafficPolicy
metadata:
namespace: default
name: policy-for-httproute-1
spec:
targetRefs:
- group: gateway.networking.k8s.io
kind: HTTPRoute
name: httproute-1
requestBuffer:
limit: 4Mi
```

Start by applying the Gateway, ListenerSet, HTTPRoute, and BackendTrafficPolicy manifests from the report and inspect the resulting policy status fields and xDS output. Trace how policies targeting the HTTPRoute are resolved when its parentRef kind is ListenerSet, then verify that the policy is no longer ignored and affects the generated configuration.

Written by the indexing model from the issue text.