cp: handle --parents source ending in parent dir

[nkgotcode]

Fixes #12626.

When cp --parents -r receives a source path that ends in .., the parent-permission copy path can reach a root path without a file name and panic. This keeps that path on the existing error path instead, matching GNU coreutils behavior for the repro case.

The regression test covers the trailing-parent-dir source and asserts the command exits with the expected directory-exists error instead of panicking or silently succeeding.

Validation:

• cargo test --features cp --test tests test_cp::test_cp_parents_recursive_source_ending_in_parent_dir -- --exact
• cargo test --features cp --test tests test_cp::test_cp_parents_2_dir -- --exact
• manual repro: coreutils cp --parents -r src/sub/.. d exits 1 with cp: cannot create directory 'd/src/sub/..': File exists, leaves d/src/sub, and does not copy src/sub/f
• _default/scripts/verify-no-ai-coauthor.sh passed immediately before push

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/pr/bounded-memory (fails in this run but passes in the 'main' branch)
Note: The gnu test tests/dd/no-allocate is now being skipped but was previously passing.
Congrats! The gnu test tests/cut/bounded-memory is now passing!
Congrats! The gnu test tests/seq/seq-epipe is now passing!

[cakebaker]

Thanks for your PR!