BackendTrafficPolicy responseOverride not applied when no HTTPRoute matches

Hi, reading the docs at https://gateway.envoyproxy.io/docs/tasks/traffic/response-override/ it seems that it should be possible to override responses generated by envoyproxy when no HTTPRoute matches.

See this example config:
```yaml
apiVersion: gateway.networking.k8s.io/v1
kind: HTTPRoute
spec:
parentRefs:
- name: development-gateway
hostnames: ["helloworld.sample.com"]
rules:
- matches:
- path:
type: RegularExpression
value: "/[Hh]ell[oO]"
backendRefs:
- name: helloworld
port: 5000
- matches:
- path:
type: PathPrefix
value: /api
backendRefs:
- name: api-backend
port: 3000
```

and this BackendTrafficPolicy
```yaml
apiVersion: gateway.envoyproxy.io/v1alpha1
kind: BackendTrafficPolicy
spec:
responseOverride:
- match:
statusCodes:
- type: Value
value: 404
response:
body:
inline: not found - 404
type: Inline
contentType: text/html
source: Local # In my opinion, 'Local' should be correct, but neither one works.
- match:
statusCodes:
- range:
end: 505
start: 500
type: Range
response:
body:
inline: Maintenance 50x
type: Inline
contentType: text/html
source: Local # In my opinion, 'Local' should be correct, but neither one works.
targetRefs:
- group: gateway.networking.k8s.io
kind: Gateway
name: development-gateway
```

A down backend (helloworld set replicas to 0) gives me the correct overridden response:
```
➜ ~ curl -v helloworld.sample.com:8080/hello
* Host helloworld.sample.com:8080 was resolved.
* IPv6: (none)
* IPv4: 127.0.0.1
* Trying 127.0.0.1:8080...
* Established connection to helloworld.sample.com (127.0.0.1 port 8080) from 127.0.0.1 port 46538
* using HTTP/1.x
> GET /hello HTTP/1.1
> Host: helloworld.sample.com:8080
> User-Agent: curl/8.18.0
> Accept: */*
>
* Request completely sent off
< HTTP/1.1 503 Service Unavailable
< content-type: text/html
< content-length: 41
< date: Thu, 21 May 2026 10:48:48 GMT
<
* Connection #0 to host helloworld.sample.com:8080 left intact
Maintenance 50x
```

but a 404, generated by envoyproxy because no HTTPRoute matches, does **not** override the response:
```
➜ ~ curl -v helloworld.sample.com:8080/non-exsistent-route
* Host helloworld.sample.com:8080 was resolved.
* IPv6: (none)
* IPv4: 127.0.0.1
* Trying 127.0.0.1:8080...
* Established connection to helloworld.sample.com (127.0.0.1 port 8080) from 127.0.0.1 port 44736
* using HTTP/1.x
> GET /non-exsistent-route HTTP/1.1
> Host: helloworld.sample.com:8080
> User-Agent: curl/8.18.0
> Accept: */*
>
* Request completely sent off
< HTTP/1.1 404 Not Found
< date: Thu, 21 May 2026 10:49:56 GMT
< content-length: 0
<
* Connection #0 to host helloworld.sample.com:8080 left intact
```
(see no `not found - 404` here)

the proxy logs this:
```
{
":authority": "helloworld.sample.com:8080",
"bytes_received": 0,
"bytes_sent": 0,
"connection_termination_details": null,
"downstream_local_address": "127.0.0.1:10080",
"downstream_remote_address": "127.0.0.1:45166",
"duration": 0,
"method": "GET",
"protocol": "HTTP/1.1",
"requested_server_name": null,
"response_code": 404,
"response_code_details": "route_not_found",
"response_flags": "NR",
"route_name": null,
"start_time": "2026-05-21T10:52:34.109Z",
"upstream_cluster": null,
"upstream_host": null,
"upstream_local_address": null,
"upstream_transport_failure_reason": null,
"user-agent": "curl/8.18.0",
"x-envoy-origin-path": "/non-exsistent-route",
"x-envoy-upstream-service-time": null,
"x-forwarded-for": "10.124.162.173",
"x-request-id": "ad40fc51-ddf5-45fa-8be5-7a08e8904110"
}
```

Of course, I could implement a custom-error-pages service the old-fashioned way, but this would be way more elegant.