fix(helpers): use asyncio.get_running_loop() inside async methods (#3289)

### What

Three `async` methods in `src/openai/helpers/` call
`asyncio.get_event_loop()` from inside a running coroutine:

- `LocalAudioPlayer.play` (`local_audio_player.py`)
- `LocalAudioPlayer.play_stream` (`local_audio_player.py`)
- `Microphone.record` (`microphone.py`)

### Why it matters

Since Python 3.10, calling `asyncio.get_event_loop()` when there is no
current event loop is deprecated, and calling it from within a coroutine
to obtain the running loop emits a `DeprecationWarning`. The documented
replacement for code that runs inside a coroutine is
`asyncio.get_running_loop()`.

### Fix

```python
# Before
loop = asyncio.get_event_loop()
# After
loop = asyncio.get_running_loop()
```

In all three cases the loop is obtained inside an `async def` and only
used for `run_in_executor` / `call_soon_threadsafe`, so there is always
a running loop and behavior is unchanged. This just removes the
deprecation and makes the intent explicit.

### Scope

Three one-line changes in `src/openai/helpers/` (a hand-maintained,
non-generated path). Files still parse and import. Happy to adjust if
you would prefer a different approach.

Co-authored-by: Marcus Wood <marcuswood@openai.com>