## Summary
Marks fixes as unsafe when they change return types (`None` → `Path`,
`str`/`bytes` → `Path`, `str` → `Path`), except when the call is a
top-level expression.
Fixes#21431.
## Problem
Fixes for `os.rename`, `os.replace`, `os.getcwd`/`os.getcwdb`, and
`os.readlink` were marked safe despite changing return types, which can
break code that uses the return value.
## Approach
Added `is_top_level_expression_call` helper to detect when a call is a
top-level expression (return value unused). Updated
`check_os_pathlib_two_arg_calls` and `check_os_pathlib_single_arg_calls`
to mark fixes as unsafe unless the call is a top-level expression.
Updated PTH109 to use the helper for applicability determination.
## Test Plan
Updated snapshots for `preview_full_name.py`, `preview_import_as.py`,
`preview_import_from.py`, and `preview_import_from_as.py` to reflect
unsafe markers.
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Co-authored-by: Brent Westbrook <brentrwestbrook@gmail.com>
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