feat(adminui): FleetDiagnosticsClient real Akka ActorSelection round-trip (F17)

- New Commons.Messages.Fleet.GetDiagnostics request record.
- DriverHostActor handles GetDiagnostics in all three states (Steady, Applying,
  Stale); replies with a NodeDiagnosticsSnapshot built from _currentRevision
  + the local NodeId. Drivers list is empty until F7 wires the per-instance
  children.
- FleetDiagnosticsClient now resolves the target via ActorSelection at
  akka.tcp://{system}@{nodeId}/user/driver-host and Asks with a 3s timeout.
  On timeout/peer-down it returns an empty snapshot so the UI degrades
  gracefully rather than throwing.

Two new integration tests in Host.IntegrationTests:
- GetDiagnostics_returns_snapshot_with_target_NodeId verifies the
  cross-node Ask/Reply works.
- GetDiagnostics_after_deploy_reports_current_revision exercises the
  end-to-end path: AdminOps starts a deployment, both DriverHostActors
  apply, then diagnostics reports the new revision on both nodes.

All 98 v2 tests pass (was 96 + 2 new).

src/Core/ZB.MOM.WW.OtOpcUa.Commons/Messages/Fleet/GetDiagnostics.cs

@@ -0,0 +1,10 @@
+using ZB.MOM.WW.OtOpcUa.Commons.Types;
+
+namespace ZB.MOM.WW.OtOpcUa.Commons.Messages.Fleet;
+
+/// <summary>
+/// Request a diagnostic snapshot from a per-node <c>DriverHostActor</c>. Sent by the admin UI's
+/// <c>IFleetDiagnosticsClient</c> via <c>ActorSelection</c> over the cluster; the local
+/// DriverHostActor responds with a <c>NodeDiagnosticsSnapshot</c>.
+/// </summary>
+public sealed record GetDiagnostics(CorrelationId CorrelationId);

src/Server/ZB.MOM.WW.OtOpcUa.AdminUI/Clients/FleetDiagnosticsClient.cs

@@ -1,36 +1,50 @@
 using Akka.Actor;
+using Microsoft.Extensions.Options;
+using ZB.MOM.WW.OtOpcUa.Cluster;
 using ZB.MOM.WW.OtOpcUa.Commons.Interfaces;
+using ZB.MOM.WW.OtOpcUa.Commons.Messages.Fleet;
 using ZB.MOM.WW.OtOpcUa.Commons.Types;
 
 namespace ZB.MOM.WW.OtOpcUa.AdminUI.Clients;
 
 /// <summary>
-/// <see cref="IFleetDiagnosticsClient"/> that targets a named node's <c>DriverHostActor</c> over
-/// Akka cluster <see cref="ActorSelection"/>.
+/// <see cref="IFleetDiagnosticsClient"/> backed by an Akka <see cref="ActorSelection"/> against
+/// the target node's <c>DriverHostActor</c> at <c>akka.tcp://otopcua@&lt;host&gt;:&lt;port&gt;/user/driver-host</c>.
+/// Sends <see cref="GetDiagnostics"/>; expects a <see cref="NodeDiagnosticsSnapshot"/> reply.
 ///
-/// The actual <c>GetDiagnosticsRequest</c>/<c>NodeDiagnosticsSnapshot</c> round-trip on the
-/// driver side is staged for follow-up F17 (depends on DriverHostActor exposing the request
-/// handler; right now it only handles DispatchDeployment). For now the client returns an empty
-/// snapshot so the UI can render a "no data yet" state.
+/// On timeout or any other failure (peer down, GetDiagnostics handler missing) returns an
+/// empty snapshot so the UI degrades to "no data" instead of throwing.
 /// </summary>
 public sealed class FleetDiagnosticsClient : IFleetDiagnosticsClient
 {
-    private readonly ActorSystem _system;
+    private static readonly TimeSpan AskTimeout = TimeSpan.FromSeconds(3);
 
-    public FleetDiagnosticsClient(ActorSystem system)
+    private readonly ActorSystem _system;
+    private readonly string _systemName;
+
+    public FleetDiagnosticsClient(ActorSystem system, IOptions<AkkaClusterOptions> options)
     {
         _system = system;
+        _systemName = options.Value.SystemName;
     }
 
-    public Task<NodeDiagnosticsSnapshot> GetDiagnosticsAsync(NodeId nodeId, CancellationToken ct)
+    public async Task<NodeDiagnosticsSnapshot> GetDiagnosticsAsync(NodeId nodeId, CancellationToken ct)
     {
-        // F17: ActorSelection at $"akka.tcp://{system}@{nodeId.Value}:4053/user/driver-host"
-        // → Ask<NodeDiagnosticsSnapshot>(new GetDiagnostics(), timeout).
-        var snapshot = new NodeDiagnosticsSnapshot(
-            nodeId,
-            CurrentRevision: null,
-            Drivers: Array.Empty<DriverInstanceDiagnostics>(),
-            AsOfUtc: DateTime.UtcNow);
-        return Task.FromResult(snapshot);
+        var selection = _system.ActorSelection($"akka.tcp://{_systemName}@{nodeId.Value}/user/driver-host");
+        try
+        {
+            using var linked = CancellationTokenSource.CreateLinkedTokenSource(ct);
+            linked.CancelAfter(AskTimeout);
+            return await selection.Ask<NodeDiagnosticsSnapshot>(
+                new GetDiagnostics(CorrelationId.NewId()), AskTimeout, linked.Token);
+        }
+        catch (Exception)
+        {
+            return new NodeDiagnosticsSnapshot(
+                nodeId,
+                CurrentRevision: null,
+                Drivers: Array.Empty<DriverInstanceDiagnostics>(),
+                AsOfUtc: DateTime.UtcNow);
+        }
     }
 }

src/Server/ZB.MOM.WW.OtOpcUa.Runtime/Drivers/DriverHostActor.cs

@@ -2,7 +2,9 @@ using Akka.Actor;
 using Akka.Cluster.Tools.PublishSubscribe;
 using Akka.Event;
 using Microsoft.EntityFrameworkCore;
+using ZB.MOM.WW.OtOpcUa.Commons.Interfaces;
 using ZB.MOM.WW.OtOpcUa.Commons.Messages.Deploy;
+using ZB.MOM.WW.OtOpcUa.Commons.Messages.Fleet;
 using ZB.MOM.WW.OtOpcUa.Commons.Types;
 using ZB.MOM.WW.OtOpcUa.Configuration;
 using ZB.MOM.WW.OtOpcUa.Configuration.Entities;
@@ -134,6 +136,7 @@ public sealed class DriverHostActor : ReceiveActor, IWithTimers
     private void Steady()
     {
         Receive<DispatchDeployment>(HandleDispatchFromSteady);
+        Receive<GetDiagnostics>(HandleGetDiagnostics);
         Receive<SubscribeAck>(_ => { /* PubSub ack */ });
     }
 
@@ -151,6 +154,7 @@ public sealed class DriverHostActor : ReceiveActor, IWithTimers
                 _localNode, msg.DeploymentId, _applyingDeploymentId);
             Self.Forward(msg); // re-deliver after we transition back
         });
+        Receive<GetDiagnostics>(HandleGetDiagnostics);
         Receive<SubscribeAck>(_ => { /* PubSub ack */ });
     }
 
@@ -160,11 +164,24 @@ public sealed class DriverHostActor : ReceiveActor, IWithTimers
         {
             _log.Warning("DriverHost {Node}: ignoring DispatchDeployment while Stale (DB unreachable)", _localNode);
         });
+        Receive<GetDiagnostics>(HandleGetDiagnostics);
         Receive<RetryConfigDbConnection>(_ => TryRecoverFromStale());
         Receive<SubscribeAck>(_ => { /* PubSub ack */ });
         Timers.StartPeriodicTimer("retry-db", RetryConfigDbConnection.Instance, ReconnectInterval);
     }
 
+    private void HandleGetDiagnostics(GetDiagnostics msg)
+    {
+        // Driver-instance children aren't spawned yet (F7); the snapshot reports an empty driver
+        // list. CurrentRevision is real — it's what the host believes is its applied revision.
+        var snapshot = new NodeDiagnosticsSnapshot(
+            NodeId: _localNode,
+            CurrentRevision: _currentRevision,
+            Drivers: Array.Empty<DriverInstanceDiagnostics>(),
+            AsOfUtc: DateTime.UtcNow);
+        Sender.Tell(snapshot);
+    }
+
     private void HandleDispatchFromSteady(DispatchDeployment msg)
     {
         if (_currentRevision is { } cur && cur == msg.RevisionHash)