test: cover string receiver contracts

[hyfdev]

Summary

• cover clean-trailing and dirty str/String normalization values and Cow variants
• verify string receivers forward explicit, ambient, cached, and unavailable cwd behavior
• prove cwd-independent absolute string calls do not read ambient cwd
• cover the Path receiver identity contract for as_path

Why

The existing path semantics were well covered through Path, but several one-line string receiver forwarding contracts were not crossed with the cases where spelling or cwd state actually changes behavior.

Eleven controlled mutations survived the existing full suite, including stripping a clean trailing separator, returning a borrowed result for dirty normalization, bypassing cached cwd, reading cwd for an absolute input, ignoring an explicit relative base, and swallowing strict cwd failures. Each new assertion in this test-only change kills at least one of those mutations.

Validation

• cargo test --locked --workspace
• cargo test --locked --workspace --features cached_current_dir
• cargo test --locked --workspace --all-features
• cargo clippy --locked --workspace --all-targets --all-features -- -D warnings
• targeted deleted-cwd, cached-cwd, string receiver, and as_path tests
• adversarial review passed; Windows literals and expected results were checked independently, with native execution delegated to the existing Windows CI job

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