kap-server dropped the server-initiated WS heartbeat and no longer emits
heartbeat_ms in server_hello, but the published v1 schema still required
it, so spec-compliant clients rejected the handshake before subscribing.
- mark heartbeat_ms optional in serverHelloPayloadSchema (advisory only)
- add a ws-control test for a server_hello without heartbeat_ms
- align kimi-web WireServerHello and the server-e2e handshake assertion