fix: backend unavailable test — use free port + ws.close(timeout=2s)

- Use _free_port() instead of hardcoded 19999 (avoids CI port conflicts)
- Add timeout=2.0 to ws.close() so close handshake never blocks >2s

ilsp/server.py

@@ -72,7 +72,7 @@ async def _ws_proxy(request: web.Request, host: str, port: int) -> web.WebSocket
         )
     except (OSError, asyncio.TimeoutError) as exc:
         logger.error("Cannot connect to LSP on %s:%d — %s", host, port, exc)
-        await ws.close(code=1011, message=b"LSP backend unavailable")
+        await ws.close(code=1011, message=b"LSP backend unavailable", timeout=2.0)
         return ws
 
     async def ws_to_tcp():