fix: preserve cached TestProbe refs across recovery

[Aaronontheweb]

Summary

• register TestProbe in the actor registry via a stable forwarding actor instead of the ephemeral underlying TestActor
• retarget that stable ref synchronously when EnsureTestActorAliveAsync recreates the underlying TestActor
• add a deterministic regression test proving startup-cached IRequiredActor<TestProbe>.ActorRef values still deliver after recovery

Why

The merged host-startup race fix repairs direct TestActor use, but actors created during WithActors can cache IRequiredActor<TestProbe>.ActorRef before recovery runs. Re-registering a new raw TestActor in ActorRegistry does not update those cached refs.

This follow-up closes that hole by giving TestProbe a stable registered identity whose target can be swapped safely after recovery.

Testing

• dotnet build src/Akka.Hosting.TestKit.Tests/Akka.Hosting.TestKit.Tests.csproj -c Release
• dotnet test src/Akka.Hosting.TestKit.Tests/Akka.Hosting.TestKit.Tests.csproj -c Release --no-build --framework net8.0 --filter "Akka.Hosting.TestKit.Tests.TestActorRecoveryRegistrySpec.Cached_required_TestProbe_reference_should_survive_TestActor_recovery|Akka.Hosting.TestKit.Tests.TestActorStartupDeadlockSpec.parallel_host_start_should_not_deadlock"