mergeBackends: a Gateway-level BackendTrafficPolicy's cluster settings are applied to merged UDP/TCP clusters regardless of protocol

*Description*:
I found this when working in https://github.com/envoyproxy/gateway/pull/9896

When `mergeBackends` is enabled, a whole-Gateway `BackendTrafficPolicy` copies its entire cluster-scoped feature set onto every merged `BackendCluster`, without looking at the backend's protocol. UDP and TCP clusters therefore receive settings that are deliberately dropped on every other code path.

Without `mergeBackends`, the IR type `ir.UDPRoute` only has `LoadBalancer` and `DNS` fields, so `healthCheck`, `circuitBreaker`, `tcpKeepalive`, `proxyProtocol`, `http2` and `useClientProtocol` from a BackendTrafficPolicy can never reach a UDP cluster.
With `mergeBackends`, those settings land in CDS for the very cluster `udp_proxy` routes to — most importantly `healthCheck.active`, which makes Envoy run TCP/HTTP health checks against a UDP-only port and can mark every endpoint unhealthy.

Expected:
the settings a merged cluster receives should match what the same policy would produce for that protocol without `mergeBackends`. Turning on cluster deduplication should not change behaviour.

*Repro steps*:
1. `EnvoyProxy` with `spec.mergeBackends: {}` attached to the GatewayClass.
2. Gateway with a UDP listener + a UDPRoute with a single `backendRef` (so the backend merges).
3. A BackendTrafficPolicy targeting the **Gateway** (no `sectionName`):

```yaml
spec:
targetRefs:
- group: gateway.networking.k8s.io
kind: Gateway
name: gateway-1
healthCheck:
active:
type: TCP
timeout: 1s
interval: 5s
unhealthyThreshold: 3
healthyThreshold: 1
tcp:
send: {type: Text, text: ping}
receive: {type: Text, text: pong}
circuitBreaker:
maxConnections: 1024
```

4. Inspect the generated cluster `service////udp`.

Observed: the cluster carries `health_checks`, `circuit_breakers`, and (with HTTP timeouts / `maxRequestsPerConnection` / `useClientProtocol`) `typed_extension_protocol_options` (`HttpProtocolOptions`) — on the cluster used by `udp_proxy`.

Expected: only load balancing and DNS, i.e. identical to the same setup without `mergeBackends`.

*Environment*:
- Envoy Gateway: reproducible on main; `mergeBackends` was added in v1.9.0 (disabled by default, experimental), so only opt-in users are affected.

*Logs*:
>Include the access logs and the Envoy logs.