Add is_disconnected property to WebSocket

[r266-tech]

Description

Adds a synchronous is_disconnected property to the WebSocket class that returns True when either the client or application state is DISCONNECTED.

Closes #2766

Motivation

When handling WebSocket connections with multiple coroutines, it's common for more than one coroutine to attempt closing the connection. Without a way to check the connection state synchronously, this leads to RuntimeError exceptions.

This property enables clean, exception-free state checking:

if not websocket.is_disconnected:
    await websocket.close()

Implementation

The property simply checks whether either client_state or application_state equals WebSocketState.DISCONNECTED — both of which are already tracked by the existing state machine in starlette/websockets.py.

Changes

• Added is_disconnected property to WebSocket class
• Added unit test test_is_disconnected_property