Replace the string-based startswith + os.sep approach with
Path.is_relative_to() (Python 3.9+, we require 3.10+). This is
the idiomatic pathlib way to check path containment — it handles
separators, case sensitivity, and the equal-path case natively
without string manipulation.
Simplified tests to match: removed the now-unnecessary
test_separator_is_os_native test since is_relative_to doesn't
depend on separator choice.
