De-flake ClusterSingletonManagerLeave2Spec: watch the proxy from a dedicated probe, as upstream does

[Aaronontheweb]

Stacked on #8566.

What changes

ClusterSingletonManagerLeave2Spec watches its singleton proxy from a dedicated probe instead of the test actor, the way Akka JVM, Pekko, and the sibling ClusterSingletonManagerLeaveSpec in this repo already do. The proxy is created on first use through a lazy async factory that registers the watch once. The file moves to the async TestKit API: every RunOn, Within, AwaitAssert, ExpectMsg, EnterBarrier, and ExpectTerminated becomes its async form, a ForEach that would have hidden an async void becomes a for loop, and the one Thread.Sleep before a negative assertion becomes a delay with a comment saying it is a deliberate soak. No assertion or timeout changes.

Why

Build 131334, Linux Artery, on a PR that touched a different spec: node first expected the "MemberRemoved" string and received the proxy's Terminated instead, at 02:36:33.32.

One event drives both messages. When first finishes leaving, the cluster publishes MemberRemoved for it. The spec's member-status listener turns that into the "MemberRemoved" string for the test actor. The proxy stops itself on the same event, and because the test actor watched the proxy, its Terminated lands in the same queue. Two independent chains race into one mailbox, the event stream fans out in hash order, and nothing orders them. The spec asserted the string first, so the notice arriving first failed it. This is a general TestKit hazard: a watch from the test actor makes every later ExpectMsg an implicit ordering assertion over Terminated as well.

The leave hand-over itself is correct; the singleton's PostStop runs before removal, as the spec's comment claims. A separate product nit found on the way is #8568: the proxy's identification-failure timer starts with a zero initial delay, so every proxy start logs a false failure warning.

How it was checked

dotnet build src/contrib/cluster/Akka.Cluster.Tools.Tests.MultiNode -c Release -warnaserror clean. The spec run three times on each transport: 5 of 5 nodes passed every time, 10 to 11 s. The grep for synchronous TestKit calls, sleeps, blocking waits, and ForEach on the file returns nothing. Test-only change. No ledger entry.