Upgrade to the Athena WebSocket gateway

Upgrade the connection to `wss` and exchange JSON messages following the `GatewayWsRequest` schema. A connection-level `GatewayWsResponse` with a `status` of `success` is sent immediately after the upgrade, followed by one response per inbound request (matched by `request_id` when provided). Actions mirror the HTTP gateway surface.

curl -X GET "https://loading/wss://your-athena-host.example.com/wss/gateway"