Fix Linux build: declare swift-nio as an explicit dependency for NIOFoundationCompat

[gestrich]

Problem

Building a package that depends on SwiftAnthropic on Linux currently fails with:

AsyncHTTPClientAdapter.swift:12:8: error: no such module 'NIOFoundationCompat'

AsyncHTTPClientAdapter.swift imports NIOFoundationCompat to use Data(buffer:) for converting the ByteBuffer returned by async-http-client into Foundation's Data. However, swift-nio — the package that provides NIOFoundationCompat — is not declared as an explicit dependency in Package.swift.

async-http-client depends on swift-nio internally, but does not re-export NIOFoundationCompat as a product. Swift Package Manager does not guarantee that modules from transitive dependencies are importable, and in practice this build fails on Linux regardless of Swift version or runner environment.

Fix

Add swift-nio as an explicit package dependency and add NIOFoundationCompat as a Linux-conditional target dependency — matching the pattern already used for AsyncHTTPClient.

The from: "2.81.0" lower bound matches the minimum swift-nio version already required by async-http-client 1.25.2+, so this introduces no new version constraints in practice.

Verification

Tested against a minimal Swift 6.2 CLI package depending on SwiftAnthropic, built on ubuntu-latest using the official swift:latest Docker container. The build fails on main and passes with this fix applied.