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);