BackendTrafficPolicy status does not report missing targetRef and leaves stale observedGeneration

### Description

When a `BackendTrafficPolicy` in Envoy Gateway `v1.8.0-rc.1` is updated to reference a nonexistent `HTTPRoute`, the policy status does not clearly report the invalid attachment.

Instead:
- the policy continues to show `Accepted=True`
- no condition indicates that the target route is missing / unresolved
- `status.ancestors[*].conditions[*].observedGeneration` remains stale and does not match the latest `metadata.generation`

This makes it hard to tell from status whether the policy is actually attached to a valid target.

### Reproducer

1. Create a kind cluster
2. Install Envoy Gateway `v1.8.0-rc.1`
3. Apply the quickstart example
4. Create a valid `BackendTrafficPolicy` targeting the quickstart `HTTPRoute`
5. Update the policy so `targetRefs.name` points to a nonexistent route

### Environment

- Envoy Gateway version: `v1.8.0-rc.1`
- Install method: Helm chart `oci://docker.io/envoyproxy/gateway-helm`
- Gateway image: `docker.io/envoyproxy/gateway:v1.8.0-rc.1`

### Initial valid policy

```yaml
apiVersion: gateway.envoyproxy.io/v1alpha1
kind: BackendTrafficPolicy
metadata:
name: quickstart-btp
namespace: default
spec:
targetRefs:
- group: gateway.networking.k8s.io
kind: HTTPRoute
name: backend
retry:
numRetries: 3
perRetry:
backOff:
baseInterval: 100ms
maxInterval: 1s
timeout: 250ms
retryOn:
triggers:
- 5xx
- gateway-error
- connect-failure
```

### Updated invalid policy

```yaml
apiVersion: gateway.envoyproxy.io/v1alpha1
kind: BackendTrafficPolicy
metadata:
name: quickstart-btp
namespace: default
spec:
targetRefs:
- group: gateway.networking.k8s.io
kind: HTTPRoute
name: does-not-exist
retry:
numRetries: 3
perRetry:
backOff:
baseInterval: 100ms
maxInterval: 1s
timeout: 250ms
retryOn:
triggers:
- 5xx
- gateway-error
- connect-failure
```

### Observed behavior

The target route does not exist:

```bash
kubectl -n default get httproute does-not-exist
# Error from server (NotFound): httproutes.gateway.networking.k8s.io "does-not-exist" not found
```

But the policy status remains:

```yaml
status:
ancestors:
- ancestorRef:
group: gateway.networking.k8s.io
kind: Gateway
name: eg
namespace: default
conditions:
- type: Accepted
status: "True"
reason: Accepted
message: Policy has been accepted.
observedGeneration: 3
```

while:

```yaml
metadata:
generation: 4
```

So:
- `Accepted=True` is still reported
- no condition indicates unresolved or missing target
- `observedGeneration` is stale (`3` vs current generation `4`)

### Expected behavior

I’d expect one of the following:
- a condition indicating the referenced target does not exist / cannot be resolved
- attachment status that clearly shows the policy is not applied to any valid target
- `observedGeneration` updated to the latest generation even if attachment fails

At minimum, status should make it obvious that the policy is not effectively attached.

### Why this matters

From the user perspective, the current status is misleading:
- the object is syntactically valid, so `Accepted=True` makes sense at one level
- but there is no visible status signal that the targetRef is broken
- stale `observedGeneration` adds more ambiguity about whether reconciliation completed

This makes debugging policy attachment failures much harder.

I believe this also happens to other xPolicies and Gateway API resources.