chore: minimal PEP 8 hygiene (unused imports/vars, trivial fixes)

[harjothkhara]

What/Why

• Remove unused imports and locals (F401/F841)
• Remove f-string prefixes without placeholders (F541)
• Keep scope minimal; avoid import reordering, mass line wrapping, naming changes in core math code, or docstring sweeps
• No functional changes; this is hygiene-only and intended to be easy to review

Files touched

• examples/gradio/gradio_chat.py
• gpt_oss/evals/aime_eval.py
• gpt_oss/responses_api/inference/triton.py
• tests/test_api_endpoints.py

Notes

• Ran Ruff locally and verified only the touched files for F/N rules to keep the change small
• Happy to follow up (in a separate PR) with small import reordering or docstring improvements if preferred

CC: @Maratyszcza @dkundel-openai

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Pull Request opened by Augment Code with guidance from the PR author

[harjothkhara]

Thanks for the great project! For clarity: I synchronized my fork with upstream/main (fast-forwarded to the latest commit) and rebased this branch to ensure the diff is minimal and current.

Scope is intentionally tiny (F401/F841/F541 only) and avoids import reordering, line wrapping, or API/naming changes. Happy to adjust per maintainer preference or split further by file if that helps the review.

cc @Maratyszcza @dkundel-openai