fix: support WebSocket proxies and Python 3.13 event loops

[yelan187]

Summary

• add an optional proxy argument to public and private WebSocket clients
• forward explicit proxies to websockets.connect
• propagate the original connection exception instead of returning None
• bind the event loop when start() runs, avoiding get_event_loop() failures on Python 3.13
• modernize WebSocket tests to use asyncio.run
• document explicit WebSocket proxy configuration

Validation

• python -m pytest test/unit/ -q — 198 passed
• verified a live demo public WebSocket subscription through an HTTP proxy and received BTC-USDT ticker messages