fix(filter): return AsyncFilterLayer from AsyncFilter::layer

[ameyypawar]

Summary

Fixes #597.

AsyncFilter::layer is documented to wrap services with an AsyncFilter built from an AsyncPredicate, but it returned FilterLayer<U> — the synchronous layer, which builds a Filter from a sync Predicate:

// before
pub fn layer(predicate: U) -> FilterLayer<U> {
    FilterLayer::new(predicate)
}
// after
pub fn layer(predicate: U) -> AsyncFilterLayer<U> {
    AsyncFilterLayer::new(predicate)
}

This is a copy/paste slip from the original filter rework (#508) — AsyncFilter::layer produced a layer that builds the wrong service type, making the constructor unusable for its stated purpose (an AsyncPredicate doesn't satisfy FilterLayer's sync Predicate bound). It now mirrors Filter::layer.

⚠️ Breaking change

This changes the public return type of AsyncFilter::layer from FilterLayer<U> to AsyncFilterLayer<U>, so it's technically a semver-breaking change. In practice no working code could have depended on the old return — a FilterLayer built from an AsyncPredicate doesn't implement Layer usefully (the predicate doesn't satisfy the sync Predicate bound). Flagging it so you can decide whether it rides a minor release or a 0.6. (Ref: the original report notes @hawkw considered it likely fine for a minor.)

Notes

• AsyncFilterLayer::new has the same signature as FilterLayer::new, and AsyncFilterLayer's Layer impl produces AsyncFilter<S, U> — so the corrected method is consistent with the rest of the module.
• Kept layer non-const to match the sibling Filter::layer (both are non-const even though the underlying *Layer::new constructors are const).
• Verified locally: cargo check --workspace --all-features --all-targets and the doc-test suite both pass, so the return-type change breaks no call sites.

I left this as the focused one-line fix; happy to add a doc/check_service-style test demonstrating AsyncFilter::layer builds a working async filter if you'd like one in this PR.