chore: Port ContractTestService from Kitura to Hummingbird

[keelerm84]

Kitura is archived and its transitive BlueSSLService uses the OpenSSL 1.1
SSL_get_peer_certificate API removed in OpenSSL 3, so the contract-test
service could not build on a modern Linux (Ubuntu 24.04 / OpenSSL 3) swift
container. Replace it with Hummingbird (swift-nio based, vendored BoringSSL).

The SDK-facing logic (CreateStreamReq, CallbackForwarder, EventSource usage) is
unchanged; only the HTTP server layer moves:

• The 4 routes port to Hummingbird's async router.
• Stream state moves to an actor (EventSource is Sendable).
• DELETE / stops the service via graceful ServiceGroup shutdown.
• URLSession/URLRequest are gated behind FoundationNetworking for Linux.

Verified: builds in the swift:6.1 Linux container (which Kitura could not) and
smoke-tested (GET / capabilities, 404/400 paths, graceful shutdown via DELETE /).
CI is unchanged: contract-tests still runs on macOS and validates the full
protocol against this service.

---

Note

Medium Risk
Full HTTP server swap for the contract harness; protocol-facing routes must stay compatible with existing macOS contract-tests despite async/concurrency changes.

Overview
Replaces the archived Kitura stack in ContractTestService with Hummingbird and the swift-server ecosystem so the contract-test binary builds on modern Linux (OpenSSL 3) while keeping the same SSE harness behavior.

Package.swift bumps to Swift 6.0, raises minimum OS versions for Hummingbird, swaps dependencies (Hummingbird, ServiceLifecycle, Logging, HTTPTypes), and pins a new Package.resolved graph (NIO/async-http-client instead of Kitura/OpenSSL 1.1).

main.swift reimplements the four control routes on Hummingbird’s async Router (ResponseEncodable, request.decode, HTTPError for 404). Active streams move from a DispatchQueue-guarded dictionary to a StreamStore actor; DELETE / triggers ServiceGroup graceful shutdown via a ShutdownHandle instead of Kitura.stop(). FoundationNetworking is imported on Linux for URLSession. CreateStreamReq, CallbackForwarder, and EventSource usage are intentionally unchanged.

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