fix(files): support PathLike objects inside upload tuples

[Rajeev91691]

Summary

This PR fixes a bug where PathLike objects passed inside custom upload tuples (e.g. ("custom_name.txt", Path("file.txt"))) were not normalized to bytes, causing httpx to raise an AttributeError at runtime (see #3473).

Changes

1. src/openai/_files.py:
   • Excluded tuple from is_file_content so that custom upload tuples fall through to the is_tuple_t block in _transform_file and _async_transform_file, which successfully normalizes the nested PathLike object.
   • Updated assert_is_file_content to check for is_file_content(obj) or isinstance(obj, tuple) to preserve tuple input validation.
2. tests/test_files.py:
   • Added test_tuple_in_mapping_normalizes_path and test_async_tuple_in_mapping_normalizes_path to verify the fix for both sync and async paths.

All 16 file tests pass successfully.

[jbeckwith-oai]

Reviewed the tuple normalization path and extraction validation behavior. This correctly restores the distinction between raw file content and multipart tuples, routes tuple contents through the existing sync/async PathLike readers, and preserves tuple acceptance during file extraction. The focused tests fail on the base commit and pass on this head; the full repository lint/typecheck/import gate and file tests are clean.