The pytest-retry plugin was causing teardown crashes due to a bug with
pytest's tmp_path fixture stash. Removing `@pytest.mark.flaky` and instead
improving the rate limit detection to properly skip tests on 429 errors.
Also fixed a brittle error message regex - GitHub changed their error
format from "tool not found" to "unknown tool".
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Co-authored-by: Claude <noreply@anthropic.com>