When `rpc.Client.Close()` is called, the TCP connection is torn down
without sending a WebSocket Close frame. The server sees `websocket:
close 1006 (abnormal closure): unexpected EOF` instead of a clean 1000
(normal closure).
### Root cause
`websocketCodec.close()` delegates to `jsonCodec.close()` which calls
`c.conn.Close()` — gorilla/websocket's `Conn.Close` explicitly "[closes
the underlying network connection without sending or waiting for a close
message](https://pkg.go.dev/github.com/gorilla/websocket#Conn.Close)"
(per RFC 6455).
### Fix
Send a WebSocket Close control frame (opcode 0x8, status 1000) before
closing the underlying connection. Uses `WriteControl` with the same
`encMu` mutex pattern already used by `pingLoop` for write
serialization, and reuses the existing `wsPingWriteTimeout` (5s)
constant.
`WriteControl` errors are safe to ignore — the connection may already be
broken by the time we attempt the close frame.
Fixes#30482
33c1bd59ff