fix(ci): satisfy Rust 1.98 chunks lint

[Garfield-yin]

Summary

• replace two constant-size chunks_exact iterators with MSRV-compatible as_chunks iteration
• preserve the existing behavior of ignoring an incomplete trailing UTF-16 code unit
• restore Clippy compatibility with Rust 1.98

Verification

• cargo +1.98.0 fmt --all -- --check
• cargo +1.98.0 clippy --workspace --all-targets -- -D warnings
• cargo +1.98.0 test --workspace
• cargo +1.88.0 check --workspace --all-targets