Motivation

• Improve unit-test coverage for the public sink::TempArtifact wrapper to exercise lifecycle behavior and encoding edge-cases.
• Ensure the wrapper's behavior mirrors the internal tempfile-backed implementation: file exists while the artifact lives and is deleted on drop, and read_to_string() preserves replacement-character semantics for invalid UTF-8.

Description

• Added imports for std::thread and std::time::Duration to support polling of filesystem cleanup in tests.
• Added temp_artifact_exposes_live_path_and_deletes_on_drop test that asserts TempArtifact::path() points to an existing file while alive and that the tempfile is removed after the wrapper is dropped.
• Added temp_artifact_read_to_string_replaces_invalid_utf8 test that verifies read_to_string() surfaces the Unicode replacement character for invalid UTF-8 bytes via the public sink::TempArtifact API.
• Kept existing round-trip tests (new_string and new_bytes) intact to preserve basic coverage.

Testing

• Ran the targeted unit test with cargo test -p uselesskey-core temp_artifact_exposes_live_path_and_deletes_on_drop -- --nocapture, which passed.
• Ran the module-level test filter with cargo test -p uselesskey-core sink::mod::tests -- --nocapture; the command executed in the CI-like workspace (initially produced zero selected tests due to filter matching), and the follow-up exact test run passed.

Codex Task