refactor: use DeprecationWarning instead of logger.warning (#687)

## Summary

- Replace `logger.warning()` with `warnings.warn(DeprecationWarning)`
for deprecated params in `batch_add_requests()` (both sync and async) to
match the pattern used in `DatasetClient.download_items()`
- `DeprecationWarning` is the Python standard — visible by default,
caught by `pytest -W error::DeprecationWarning`, and filterable via
`warnings.filterwarnings()`
- Remove unused `logging` import

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Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>