Gateway-wide CTP incorrectly accepted when Gateway name is a prefix of another Gateway name (mergeGateways)

**Description**:

When `mergeGateways` is enabled and two Gateways exist where one name is a string prefix of the other (e.g. `gw` and `gw-b`), a Gateway-wide `ClientTrafficPolicy` targeting the shorter-named Gateway is incorrectly accepted even though a same-port HTTP listener exists on the other Gateway. The port overlap validation in `validatePortOverlapForClientTrafficPolicy` uses `strings.Index(currName, prefix) == 0` to check whether a same-port listener belongs to the targeted Gateway, but because `extractGatewayNameFromListener` returns the prefix without a trailing slash, `"envoy-gateway/gw"` falsely matches `"envoy-gateway/gw-b/http"`, causing the conflict to go undetected.

The same bug was identified and fixed for `extractListenerSetPrefixFromListener` in #9026 (trailing slash added to the returned prefix). The Gateway path has the same vulnerability.

**Repro steps**:

Apply the following resources:

```yaml
envoyProxyForGatewayClass:
apiVersion: gateway.envoyproxy.io/v1alpha1
kind: EnvoyProxy
metadata:
namespace: envoy-gateway
name: test
spec:
mergeGateways: true
clientTrafficPolicies:
- apiVersion: gateway.envoyproxy.io/v1alpha1
kind: ClientTrafficPolicy
metadata:
namespace: envoy-gateway
name: ctp-gw-wide
spec:
targetRefs:
- group: gateway.networking.k8s.io
kind: Gateway
name: gw
tcpKeepalive:
probes: 5
gateways:
- apiVersion: gateway.networking.k8s.io/v1
kind: Gateway
metadata:
namespace: envoy-gateway
name: gw
spec:
gatewayClassName: envoy-gateway-class
listeners:
- name: http
hostname: gw.example.com
protocol: HTTP
port: 8081
allowedRoutes:
namespaces:
from: Same
- apiVersion: gateway.networking.k8s.io/v1
kind: Gateway
metadata:
namespace: envoy-gateway
name: gw-b
spec:
gatewayClassName: envoy-gateway-class
listeners:
- name: http
hostname: gw-b.example.com
protocol: HTTP
port: 8081
allowedRoutes:
namespaces:
from: Same
```

Observe that the `ClientTrafficPolicy` status shows `Accepted: True` — it should show a port conflict error referencing the `gw-b` listener.

**Environment**:

Reproducible via the translation unit tests, no cluster required. The test fixture is at `internal/gatewayapi/testdata/clienttrafficpolicy-gateway-wide-same-port-prefix-name-conflict.{in,out}.yaml`.

**Logs**:

N/A