rpc: send WebSocket close frame on client disconnect (#33909)

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

rpc/websocket.go

@@ -324,6 +324,16 @@ func newWebsocketCodec(conn *websocket.Conn, host string, req http.Header, readL
 }
 
 func (wc *websocketCodec) close() {
+	// Send a WebSocket Close frame before closing the underlying connection,
+	// so the server sees a clean 1000 (normal closure) instead of 1006 (abnormal).
+	wc.jsonCodec.encMu.Lock()
+	wc.conn.WriteControl(
+		websocket.CloseMessage,
+		websocket.FormatCloseMessage(websocket.CloseNormalClosure, ""),
+		time.Now().Add(wsPingWriteTimeout),
+	)
+	wc.jsonCodec.encMu.Unlock()
+
 	wc.jsonCodec.close()
 	wc.wg.Wait()
 }