test(host): 2-node integration test harness + consolidate to one ActorSystem (Task 58)

Builds TwoNodeClusterHarness: two in-process Host-equivalent nodes sharing
an in-memory ConfigDb. Forms a 2-member Akka cluster. ClusterFormationTests
proves both nodes see each other as admin+driver role members.

Fixes a real production bug uncovered while wiring the harness — Program.cs
ran two separate ActorSystems (one from AddOtOpcUaCluster.AkkaHostedService
with cluster HOCON, one from Akka.Hosting.AddAkka with bare HOCON). Cluster
singletons landed on the bare ActorSystem and could not actually form a
cluster ("Configuration does not contain `akka.cluster` node").

Consolidation:
- AddOtOpcUaCluster now only binds AkkaClusterOptions + registers IClusterRoleInfo
- New WithOtOpcUaClusterBootstrap pushes embedded HOCON + Remote/Cluster options
  into Akka.Hosting's AkkaConfigurationBuilder
- AkkaHostedService.cs deleted — Akka.Hosting now owns the lifecycle
- Program.cs + harness call WithOtOpcUaClusterBootstrap inside AddAkka

Why not WebApplicationFactory<Program>? Program.cs reads OTOPCUA_ROLES from
process env (shared across in-process WAFs); the harness replays Program.cs's
DI graph from a clean WebApplicationBuilder per node with per-node config
overrides. Same production extensions, isolated config + Kestrel + Akka ports.

Tests: 93 v2 tests pass (was 91 + 2 new cluster formation), 0 skipped.

src/Server/ZB.MOM.WW.OtOpcUa.Host/Program.cs

@@ -40,8 +40,9 @@ builder.Services.AddOtOpcUaCluster(builder.Configuration);
 
 // Akka cluster bootstrap. Role-specific singletons are registered on the AkkaConfigurationBuilder
 // from inside the configurator lambda. AddAkka spins the ActorSystem at host start.
-builder.Services.AddAkka("otopcua", (ab, _) =>
+builder.Services.AddAkka("otopcua", (ab, sp) =>
 {
+    ab.WithOtOpcUaClusterBootstrap(sp);
     if (hasAdmin)
         ab.WithOtOpcUaControlPlaneSingletons();
     if (hasDriver)