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).

tests/Server/ZB.MOM.WW.OtOpcUa.Host.IntegrationTests/FleetDiagnosticsRoundTripTests.cs

@@ -0,0 +1,81 @@
+using Microsoft.Extensions.DependencyInjection;
+using Shouldly;
+using Xunit;
+using ZB.MOM.WW.OtOpcUa.Commons.Interfaces;
+using ZB.MOM.WW.OtOpcUa.Commons.Messages.Admin;
+
+namespace ZB.MOM.WW.OtOpcUa.Host.IntegrationTests;
+
+/// <summary>
+/// End-to-end <see cref="IFleetDiagnosticsClient"/> round-trip via the cluster:
+/// admin node asks node-B's <c>DriverHostActor</c> for a snapshot via <c>ActorSelection</c>.
+/// Verifies the cross-node Ask/Reply works and the snapshot reflects the target node's
+/// view (NodeId + CurrentRevision after a deploy).
+/// </summary>
+public sealed class FleetDiagnosticsRoundTripTests
+{
+    private static CancellationToken Ct => TestContext.Current.CancellationToken;
+
+    [Fact]
+    public async Task GetDiagnostics_returns_snapshot_with_target_NodeId()
+    {
+        await using var harness = await TwoNodeClusterHarness.StartAsync();
+
+        // Resolve target NodeId from the cluster — second member ordered by address.
+        var members = Akka.Cluster.Cluster.Get(harness.NodeASystem).State.Members
+            .OrderBy(m => m.Address.ToString())
+            .ToArray();
+        members.Length.ShouldBe(2);
+        var targetAddress = members[1].Address;
+        var targetNodeId = Commons.Types.NodeId.Parse($"{targetAddress.Host}:{targetAddress.Port}");
+
+        await using var scope = harness.NodeA.Services.CreateAsyncScope();
+        var client = scope.ServiceProvider.GetRequiredService<IFleetDiagnosticsClient>();
+
+        var snapshot = await client.GetDiagnosticsAsync(targetNodeId, Ct);
+
+        snapshot.NodeId.ShouldBe(targetNodeId);
+        snapshot.Drivers.ShouldBeEmpty();   // No driver children yet (F7).
+        snapshot.AsOfUtc.ShouldBeGreaterThan(DateTime.UtcNow.AddSeconds(-30));
+    }
+
+    [Fact]
+    public async Task GetDiagnostics_after_deploy_reports_current_revision()
+    {
+        await using var harness = await TwoNodeClusterHarness.StartAsync();
+
+        await using var scope = harness.NodeA.Services.CreateAsyncScope();
+        var adminOps = scope.ServiceProvider.GetRequiredService<IAdminOperationsClient>();
+        var diagnostics = scope.ServiceProvider.GetRequiredService<IFleetDiagnosticsClient>();
+
+        var deploy = await adminOps.StartDeploymentAsync(createdBy: "alice@test", Ct);
+        deploy.Outcome.ShouldBe(StartDeploymentOutcome.Accepted);
+        var expectedRev = deploy.RevisionHash!.Value;
+
+        // Wait until both DriverHostActors have caught up to the deployed revision.
+        var members = Akka.Cluster.Cluster.Get(harness.NodeASystem).State.Members
+            .OrderBy(m => m.Address.ToString())
+            .Select(m => Commons.Types.NodeId.Parse($"{m.Address.Host}:{m.Address.Port}"))
+            .ToArray();
+
+        foreach (var nodeId in members)
+        {
+            await WaitForAsync(async () =>
+            {
+                var snap = await diagnostics.GetDiagnosticsAsync(nodeId, Ct);
+                return snap.CurrentRevision == expectedRev;
+            }, TimeSpan.FromSeconds(15));
+        }
+    }
+
+    private static async Task WaitForAsync(Func<Task<bool>> condition, TimeSpan timeout)
+    {
+        var deadline = DateTime.UtcNow + timeout;
+        while (DateTime.UtcNow < deadline)
+        {
+            if (await condition()) return;
+            await Task.Delay(200);
+        }
+        throw new TimeoutException($"Condition not met within {timeout}");
+    }
+}