fix: remove double-base64 encoding in upload_file/download_file

[Hweinstock]

Issue

Fixes #458

Description

upload_file/upload_files redundantly base64-encode bytes before passing to botocore's blob shape (which encodes again). download_file/download_files redundantly base64-decode the response blob that botocore already decoded. Binary files are corrupted on upload and crash or return garbage on download.

Changes

• Remove base64.b64encode() in upload_file/upload_files — pass raw bytes to botocore
• Remove base64.b64decode() in download_file/download_files — use botocore's already-decoded bytes
• Remove unused import base64
• Update unit tests to assert correct behavior (raw bytes, not pre-encoded strings)
• Add integration test verifying binary roundtrip (test_code_interpreter_client.py)

Testing

• Unit tests: uv run pytest tests/ -k code_interpreter — all pass
• Integration tests: uv run pytest tests_integ/tools/test_code_interpreter_client.py -xvs — binary upload/download roundtrip verified against live service

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
⚠️ Please upload report for BASE (main@d69db93). Learn more about missing BASE report.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #464   +/-   ##
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  Coverage        ?   89.40%           
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  Files           ?       82           
  Lines           ?     7570           
  Branches        ?     1132           
=======================================
  Hits            ?     6768           
  Misses          ?      511           
  Partials        ?      291           

Flag	Coverage Δ
unittests	89.40% <100.00%> (?)

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