chore: allow unused async trait handlers

[lucarlig]

Summary

• allow clippy::unused_async_trait_impl at workspace scope
• keep the existing async trait implementations unchanged

This is the one-line bottom PR beneath #107 and #109.

Why this lint is allowed

Clippy's unused_async_trait_impl documentation explains that the lint flags async trait-method implementations with no .await. It was added in Rust 1.98 as a pedantic lint and recommends returning core::future::ready to reduce the generated future state machine.

The same documentation notes that the rewrite is not behaviorally identical: future::ready computes the value immediately, while an async method defers computation until the future is polled. This workspace enables pedantic Clippy lints and denies warnings in CI, so Rust 1.98 turned these existing interface implementations into a hard failure. We keep their current evaluation behavior and suppress this specific lint centrally instead of rewriting production and test handlers.

Verification

• Rust 1.98 workspace Clippy with warnings denied
• Rust 1.96 workspace Clippy completes with its expected unknown-lint compatibility warning
• repository commit hooks: fmt, Clippy, deny, nextest, and build