codex: use camino for UTF-8 path assertions

[hardfist]

What changed

• Adds camino and introduces path_to_utf8(&Path) -> &Utf8Path as the central UTF-8 path assertion helper.
• Reworks internal UTF-8 path string access in resolver and tsconfig code to go through Utf8Path::as_str().
• Keeps the public API on std::path for now, so this stacked PR avoids a breaking API migration.

Why

This builds on #219 by making the UTF-8-only path invariant explicit with a dedicated UTF-8 path type instead of ad hoc Path::to_str() assertions.

Scope

This is intentionally a low-risk first camino step. It does not migrate public PathBuf/Path API types, filesystem traits, errors, or option structs to Utf8PathBuf yet. Those can be handled in follow-up PRs if we want a full public API migration.

Validation

• cargo fmt --all --check
• cargo check --workspace
• cargo clippy --all-features -- -D warnings
• cargo test -p rspack_resolver --lib -- --skip pnp

[codspeed-hq]

Merging this PR will not alter performance

✅ 12 untouched benchmarks

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_{Comparing codex/use-camino-utf8-path (ccc0343) with main (0616bdc)}