fix(helpers): use running loop in voice helpers

[Ghraven]

Problem

The voice helper async methods are already running inside coroutines, but they call asyncio.get_event_loop() before using the loop for thread-safe callbacks and executor work. On modern Python, asyncio.get_running_loop() is the clearer API in this context and avoids depending on event-loop policy fallback behavior.

Before / after

Before, Microphone.record(), LocalAudioPlayer.play(), and LocalAudioPlayer.play_stream() fetched the event loop with get_event_loop().

After, those async methods capture the currently running loop with get_running_loop() while keeping the existing call_soon_threadsafe() and run_in_executor() behavior unchanged.

Verification

• python -m py_compile src/openai/helpers/microphone.py src/openai/helpers/local_audio_player.py
• git diff --check

I did not run a hardware audio smoke test locally.