Handle async proxy calls after event loop closes

[Wackymax]

What changed

• make coroutine methods exposed by ThreadsafeProxy consistently return an awaitable wrapper
• treat calls made after the worker event loop has closed as a disconnected no-op
• add a regression test covering an async proxy method after loop closure

Why

When a threaded EZSP transport loses its connection, the worker event loop can already be closed by the time ZHA asks bellows to disconnect. The existing proxy logs Attempted to use a closed event loop and returns plain None, even when the proxied method is asynchronous. The caller then attempts to await that value and raises:

TypeError: object NoneType can't be used in 'await' expression

This secondary cleanup exception can prevent Home Assistant's ZHA config-entry reload from recovering after the original transport failure.

The async wrapper now remains awaitable in every state. If the target loop is closed, awaiting it returns None, matching the existing disconnected/no-op behavior without raising a second exception.

Related reports:

• ZHA - application controller is not running home-assistant/core#130548
• Controller doesnt reconnect after serial connection lost. #654

Validation

• added test_proxy_async_loop_closed
• python -m pytest -q: 425 passed
• git diff --check