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## Summary
<!-- What's the purpose of the change? What does it do, and why? -->
Fixes#20255
Mark single-item-membership-test fixes as always unsafe
- Always set `Applicability::Unsafe` for FURB171 fixes
- Update “Fix safety” docs to reflect always-unsafe behavior
- Expand tests (not in, nested set/frozenset, commented args)
## Test Plan
<!-- How was it tested? -->
I have added new test cases to
`crates/ruff_linter/resources/test/fixtures/refurb/FURB171_0.py` and
`crates/ruff_linter/resources/test/fixtures/refurb/FURB171_1.py`.
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Co-authored-by: Brent Westbrook <36778786+ntBre@users.noreply.github.com>
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