fix(ci): support Rust 1.98 clippy

[jiunbae]

Summary

• allow the Rust 1.98 name for the intentionally async AskStore::load API while retaining compatibility with older toolchains
• remove a test-only import that became redundant under Rust 1.98
• use fixed-size byte chunks when decoding relay attach tokens

Why

The post-merge main CI run for #77 picked up Rust 1.98 and failed on newly enabled Clippy lints, although the PR run had passed on Rust 1.96. This keeps main green across the project MSRV and current stable.

Verification

• cargo fmt --all -- --check
• cargo clippy --workspace --all-targets --locked -- -D warnings (Rust 1.96)
• cargo +1.98.0 fmt --all -- --check
• cargo +1.98.0 clippy --workspace --all-targets --locked -- -D warnings
• cargo +1.88.0 check --workspace --locked
• cargo build --workspace --bins --locked && cargo test --workspace --locked