fix(lib): use log.warning instead of the deprecated log.warn (#3878)

## Summary

`src/openai/lib/_parsing/_audio.py:35` uses the deprecated
`logging.Logger.warn` alias (emits `DeprecationWarning`); the twin
function at line 56 and `_completions.py:128` already use `log.warning`.
Aligned. This is the only `log.warn` occurrence in the SDK, in a
hand-written generator-safe module.

## Checklist

- [x] I understand that this repository is auto-generated and my pull
request may not be merged

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