feat: bridge ActorSystem into DI (transient) for shared health checks

src/ZB.MOM.WW.ScadaBridge.Host/Program.cs

@@ -120,6 +120,13 @@ try
         builder.Services.AddSingleton<AkkaHostedService>();
         builder.Services.AddHostedService(sp => sp.GetRequiredService<AkkaHostedService>());
 
+        // The shared ZB.MOM.WW.Health Akka checks resolve ActorSystem from DI. ScadaBridge owns the
+        // ActorSystem inside AkkaHostedService (not a DI singleton), so bridge it as TRANSIENT: each
+        // resolve re-reads the current value — null while warming up (checks → Degraded), live after.
+        // The factory must NOT throw: GetService<ActorSystem>() must return null (not raise) pre-start.
+        builder.Services.AddTransient<Akka.Actor.ActorSystem>(sp =>
+            sp.GetRequiredService<AkkaHostedService>().ActorSystem!);
+
         // InboundAPI-022: register the production IActiveNodeGate implementation so
         // standby-node gating is actually enforced (the InboundApiEndpointFilter
         // consults IActiveNodeGate and defaults to "allow" when none is registered,

src/ZB.MOM.WW.ScadaBridge.Host/SiteServiceRegistration.cs

@@ -73,6 +73,13 @@ public static class SiteServiceRegistration
         services.AddSingleton<AkkaHostedService>();
         services.AddHostedService(sp => sp.GetRequiredService<AkkaHostedService>());
 
+        // The shared ZB.MOM.WW.Health Akka checks resolve ActorSystem from DI. ScadaBridge owns the
+        // ActorSystem inside AkkaHostedService (not a DI singleton), so bridge it as TRANSIENT: each
+        // resolve re-reads the current value — null while warming up (checks → Degraded), live after.
+        // The factory must NOT throw: GetService<ActorSystem>() must return null (not raise) pre-start.
+        services.AddTransient<Akka.Actor.ActorSystem>(sp =>
+            sp.GetRequiredService<AkkaHostedService>().ActorSystem!);
+
         // Cluster node status provider for health reports
         services.AddSingleton<IClusterNodeProvider>(sp =>
         {

tests/ZB.MOM.WW.ScadaBridge.Host.Tests/ActorSystemBridgeTests.cs

@@ -0,0 +1,46 @@
+using Akka.Actor;
+using Microsoft.Extensions.DependencyInjection;
+using Microsoft.Extensions.Logging;
+using Microsoft.Extensions.Logging.Abstractions;
+using Microsoft.Extensions.Options;
+using ZB.MOM.WW.ScadaBridge.ClusterInfrastructure;
+using ZB.MOM.WW.ScadaBridge.Communication;
+using ZB.MOM.WW.ScadaBridge.Host;
+using ZB.MOM.WW.ScadaBridge.Host.Actors;
+
+namespace ZB.MOM.WW.ScadaBridge.Host.Tests;
+
+/// <summary>
+/// Verifies the DI bridge that exposes the Akka <see cref="ActorSystem"/> — owned by
+/// <see cref="AkkaHostedService"/>, not registered as a DI singleton — to consumers that
+/// resolve <c>ActorSystem</c> from the container (notably the shared ZB.MOM.WW.Health Akka
+/// checks). The bridge is registered TRANSIENT so each resolve re-reads the current value:
+/// null while the hosted service is warming up (checks treat that as Degraded), the live
+/// system afterwards. A SINGLETON would cache the startup-time null forever.
+/// </summary>
+public sealed class ActorSystemBridgeTests
+{
+    [Fact]
+    public void ActorSystem_ResolvesNull_BeforeHostedServiceStarts()
+    {
+        var services = new ServiceCollection();
+
+        // Register AkkaHostedService the same way Program.cs does, supplying the minimal
+        // constructor dependencies so the container can build it. Its ActorSystem property
+        // is null until StartAsync runs — which it never does here.
+        services.AddSingleton(Options.Create(new NodeOptions()));
+        services.AddSingleton(Options.Create(new ClusterOptions()));
+        services.AddSingleton(Options.Create(new CommunicationOptions()));
+        services.AddSingleton<ILogger<AkkaHostedService>>(NullLogger<AkkaHostedService>.Instance);
+        services.AddSingleton<AkkaHostedService>();
+
+        // The bridge under test: TRANSIENT factory that re-reads the owned ActorSystem.
+        services.AddTransient<ActorSystem>(sp =>
+            sp.GetRequiredService<AkkaHostedService>().ActorSystem!);
+
+        using var provider = services.BuildServiceProvider();
+
+        // The hosted service has not started, so the bridge must yield null (not throw).
+        Assert.Null(provider.GetService<ActorSystem>());
+    }
+}