test(bytestring): expand test coverage

[robjtede]

Summary

• add coverage for ByteString::into_bytes
• exercise empty, Unicode, and invalid UTF-8 conversion paths
• cover owned and borrowed byte-array conversions

Tests

• cargo llvm-cov --package bytestring --all-features --summary-only — 100% regions, functions, and lines
• cargo llvm-cov --package bytestring --no-default-features --summary-only — 100% regions, functions, and lines
• cargo test --package bytestring --all-features
• cargo +nightly fmt --package bytestring -- --check

The repository-wide just check remains blocked by existing formatting errors in deny.toml and actix-tls/Cargo.toml.