Close the async ECS discovery clients on coordinated shutdown

[pjfanning]

Motivation

Follow-up to #907, which added CoordinatedShutdown client-close tasks to the deprecated pekko-discovery-aws-api module but left pekko-discovery-aws-api-async — the module the deprecation warning tells users to migrate to — untouched. Both AsyncEcsServiceDiscovery and AsyncEcsTaskSetDiscovery build an EcsAsyncClient and never close it.

The leak is bigger here than the one that was fixed. AsyncEcsServiceDiscovery does:

val httpClient = NettyNioAsyncHttpClient.create()
EcsAsyncClient.builder().overrideConfiguration(conf).httpClient(httpClient).build()

In AWS SDK v2, a client passed to .httpClient(...) is treated as caller-owned and wrapped in NonManagedSdkAsyncHttpClient, whose close() is a no-op. So the Netty event loop threads would survive even if the ECS client were closed. .httpClientBuilder(...) is the form that hands ownership to the SDK.

Modification

• Both classes get an AtomicReference populated only after the client is built, plus a PhaseServiceUnbind task that closes whatever client it finds there and returns Done when it is null — so shutdown never forces, or re-attempts, the lazy initialisation. (Same shape as Only close AWS discovery clients that were actually built #954, which fixes that ordering bug in the v1 module.)
• The close runs on pekko.actor.default-blocking-io-dispatcher, since closing a Netty-backed client blocks until its event loops have shut down.
• AsyncEcsServiceDiscovery switches to .httpClientBuilder(NettyNioAsyncHttpClient.builder()) so the SDK owns the Netty client and shuts its event loops down on close().
• Client construction moves behind private[ecs] def createEcsClient(), so the spec can supply a stub without AWS credentials, a region, or network access. Both classes are @ApiMayChange and non-final; MiMa is clean.
• Docs note that the clients are created lazily and closed at service-unbind.

Result

Terminating the ActorSystem releases the ECS clients and their Netty event loop threads. Discovery that is configured but never used still creates no client.

Tests

• sbt "discovery-aws-api-async/test" — 4 succeeded, 0 failed. New AsyncEcsClientShutdownSpec; this module had no test sources at all before this PR.
• Directionality: the two "close the ECS client on coordinated shutdown" cases assert close() was called, which nothing on main does — there is no shutdown task there. I could not run them verbatim against main because the createEcsClient seam they stub does not exist there.
• sbt "discovery-aws-api-async/mimaReportBinaryIssues" — success.
• sbt "discovery-aws-api-async/scalafmt" "discovery-aws-api-async/Test/scalafmt" and sbt headerCreateAll — clean.
• Not exercised against live AWS — no account available. The spec stubs EcsAsyncClient so no credentials, region or network access are needed.

References

Refs #907, Refs #954