Cover MkdirAll, WriteFile, and Rename error paths (46.7% to 86.7%)
Tests the error logging path when os.ReadFile fails with an error other than ErrNotExist. Uses a directory in place of the file.
20 test cases covering: - Token generation (uniqueness, expiry durations, format) - Token validation (valid, invalid, expired, used, empty store) - Concurrent use protection (replay attack prevention) - Cleanup routine (expired tokens, used tokens, stop signal) - Persistence (save, load, filter expired on load) - Edge cases (missing file, invalid JSON)