Listener-scoped ClientTrafficPolicy settings are silently dropped when multiple listeners share an address:port

*Follow up issue from: https://github.com/envoyproxy/gateway/pull/9315#discussion_r3703983029*

*Description*:

When multiple Gateway listeners share an address:port, they collapse into a
single xDS Listener with one filter chain per section. ClientTrafficPolicy
fields that map to Listener-scoped Envoy config have only one value available
for the whole socket:

- `ConnectionInspectionTimeout` -> `listener_filters_timeout`
- `Connection.BufferLimit` -> `per_connection_buffer_limit_bytes`
- `TCPKeepalive` -> `tcp_keepalive`
- `EnableProxyProtocol` -> proxy protocol listener filter

Since CTP is tracked one-per-section, both policies are Accepted, the
first-processed value wins for the entire socket, and the other is dropped with
no status condition, warning, or log line.

The existing port-overlap check skips TLS listeners, assuming each gets its own
filter chain, which is true for filter-chain-scoped settings, but not listener-scoped ones.
See the existing [TODO in `internal/xds/translator/translator.go`](https://github.com/envoyproxy/gateway/blob/0e7757390e923a6ec64bc99ecdc23a84800ad455/internal/xds/translator/translator.go#L864).

Expected: a cross-listener consistency check that rejects conflicting
listener-scoped values across sections sharing an address:port, surfacing the
conflict via status rather than resolving it silently.

*Repro steps*:

1. Gateway with two HTTPS listeners on `:443` (different hostnames, own certs).
2. One ClientTrafficPolicy per section with differing
`Connection.BufferLimit` values (e.g. `29Mi` and `300Mi`).
3. Both policies report `Accepted=True`.
4. `envoy config_dump` shows a single `per_connection_buffer_limit_bytes` on `:443` --> one
value was dropped.

Read the TODO in internal/xds/translator/translator.go and trace how ClientTrafficPolicy values are collected for listeners sharing an address:port. Reproduce the case with two HTTPS listeners and differing Connection.BufferLimit values, then verify that conflicting listener-scoped settings are rejected through status rather than silently dropped.

Written by the indexing model from the issue text.