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/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)