chore(tests): mark sample_file.txt as text with LF line endings (#3879)

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

## Changes being requested

Add a `.gitattributes` marking `tests/sample_file.txt -text`.

A default Windows checkout (`core.autocrlf=true`, the setting Git for
Windows recommends) converts that fixture to CRLF because nothing tells
Git it is a fixed byte fixture. `test_base64_file_input` base64-encodes
the file and compares against a fixed LF-based digest, so it fails out
of the box on a fresh Windows clone:

    AssertionError: assert '...IHdvcmxkIQ0K' == '...IHdvcmxkIQo='

With the attribute, `git ls-files --eol tests/sample_file.txt` shows
`i/lf w/lf attr/-text` and `pytest tests/test_transform.py -k
base64_file_input` goes from 2 failed to 2 passed on the same clone; the
rest of `tests/test_transform.py` (60 tests) stays green. The fixture's
bytes in the repository are unchanged.

## Additional context & links

The file is not generated code, so nothing in the SDK regeneration
touches it. Same shape as other repos' fixture handling for byte-exact
test inputs.

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