[`flake8-use-pathlib`] Make `PTH111` fix unsafe because it can change behavior (#20215)

## Summary

Fixes https://github.com/astral-sh/ruff/issues/20214

## Test Plan

crates/ruff_linter/src/rules/flake8_use_pathlib/rules/os_path_expanduser.rs

@@ -1,3 +1,4 @@
+use ruff_diagnostics::Applicability;
 use ruff_macros::{ViolationMetadata, derive_message_formats};
 use ruff_python_ast::ExprCall;
 
@@ -35,7 +36,10 @@ use crate::{FixAvailability, Violation};
 /// especially on older versions of Python.
 ///
 /// ## Fix Safety
-/// This rule's fix is marked as unsafe if the replacement would remove comments attached to the original expression.
+/// This rule's fix is always marked as unsafe because the behaviors of
+/// `os.path.expanduser` and `Path.expanduser` differ when a user's home
+/// directory can't be resolved: `os.path.expanduser` returns the
+/// input unchanged, while `Path.expanduser` raises `RuntimeError`.
 ///
 /// ## References
 /// - [Python documentation: `Path.expanduser`](https://docs.python.org/3/library/pathlib.html#pathlib.Path.expanduser)
@@ -71,6 +75,6 @@ pub(crate) fn os_path_expanduser(checker: &Checker, call: &ExprCall, segments: &
         "path",
         is_fix_os_path_expanduser_enabled(checker.settings()),
         OsPathExpanduser,
-        None,
+        Some(Applicability::Unsafe),
     );
 }

crates/ruff_linter/src/rules/flake8_use_pathlib/snapshots/ruff_linter__rules__flake8_use_pathlib__tests__preview_full_name.py.snap

@@ -260,6 +260,7 @@ help: Replace with `Path(...).expanduser()`
 20 | bbb = os.path.isdir(p)
 21 | bbbb = os.path.isfile(p)
 22 | bbbbb = os.path.islink(p)
+note: This is an unsafe fix and may change runtime behavior
 
 PTH112 [*] `os.path.isdir()` should be replaced by `Path.is_dir()`
   --> full_name.py:19:7

crates/ruff_linter/src/rules/flake8_use_pathlib/snapshots/ruff_linter__rules__flake8_use_pathlib__tests__preview_import_as.py.snap

@@ -260,6 +260,7 @@ help: Replace with `Path(...).expanduser()`
 20 | bbb = foo_p.isdir(p)
 21 | bbbb = foo_p.isfile(p)
 22 | bbbbb = foo_p.islink(p)
+note: This is an unsafe fix and may change runtime behavior
 
 PTH112 [*] `os.path.isdir()` should be replaced by `Path.is_dir()`
   --> import_as.py:19:7

crates/ruff_linter/src/rules/flake8_use_pathlib/snapshots/ruff_linter__rules__flake8_use_pathlib__tests__preview_import_from.py.snap

@@ -268,6 +268,7 @@ help: Replace with `Path(...).expanduser()`
 22 | bbb = isdir(p)
 23 | bbbb = isfile(p)
 24 | bbbbb = islink(p)
+note: This is an unsafe fix and may change runtime behavior
 
 PTH112 [*] `os.path.isdir()` should be replaced by `Path.is_dir()`
   --> import_from.py:21:7

crates/ruff_linter/src/rules/flake8_use_pathlib/snapshots/ruff_linter__rules__flake8_use_pathlib__tests__preview_import_from_as.py.snap

@@ -268,6 +268,7 @@ help: Replace with `Path(...).expanduser()`
 27 | bbb = xisdir(p)
 28 | bbbb = xisfile(p)
 29 | bbbbb = xislink(p)
+note: This is an unsafe fix and may change runtime behavior
 
 PTH112 [*] `os.path.isdir()` should be replaced by `Path.is_dir()`
   --> import_from_as.py:26:7