chore: improve cargo maintenance path

[vip892766gma]

Summary:

• Add focused unit tests in crates/cargo-util/src/paths.rs covering edge cases of existing helpers like strip_prefix_canonical, normalize_path, and bytes2path (empty input, trailing separators, parent-traversal beyond root, non-UTF8 components on unix) to harden error branches without changing behavior.
• Keep the change narrow so it is straightforward to review.

Notes:

• I kept this scoped to the relevant implementation and tests.

[rustbot]

r? @weihanglo

rustbot has assigned @weihanglo.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

Why was this reviewer chosen?

The reviewer was selected based on:

• Owners of files modified in this PR: @ehuss, @epage, @weihanglo
• @ehuss, @epage, @weihanglo expanded to ehuss, epage, weihanglo
• Random selection from ehuss, epage, weihanglo