Phase 3 PR 69 -- OPC UA Client ISubscribable + IHostConnectivityProbe. Completes the OpcUaClientDriver capability surface — now matches the Galaxy + Modbus + S7 driver coverage. ISubscribable: SubscribeAsync creates a new upstream Subscription via the non-obsolete Subscription(ITelemetryContext, SubscriptionOptions) ctor + AddItem/CreateItemsAsync flow, wires each MonitoredItem's Notification event into OnDataChange. Tag strings round-trip through MonitoredItem.Handle so the notification handler can identify which tag changed without a second lookup. Publishing interval floored at 50ms (servers negotiate up anyway; sub-50ms wastes round-trip). SubscriptionOptions uses KeepAliveCount=10, LifetimeCount=1000, TimestampsToReturn=Both so SourceTimestamp passthrough for the cascading-quality rule works through subscription paths too. UnsubscribeAsync calls Subscription.DeleteAsync(silent:true) and tolerates unknown handles (returns cleanly) because the caller's race with server-side cleanup after a session drop shouldn't crash either side. Session shutdown explicitly deletes every remote subscription before closing — avoids BadSubscriptionIdInvalid noise in the upstream server's log on Close. IHostConnectivityProbe: HostName surfaced as the EndpointUrl (not host:port like the Modbus/S7 drivers) so the Admin /hosts dashboard can render the full opc.tcp:// URL as a clickable target back at the remote server. HostState tracked via session.KeepAlive event — OPC UA's built-in keep-alive is authoritative for session liveness (the SDK pings on KeepAliveInterval, sets KeepAliveStopped after N missed pings), strictly better than a driver-side polling probe: no extra wire round-trip, no duplicate semantic with the native protocol. Handler transitions Running on healthy keep-alives and Stopped on any Bad service-result. Initial Running raised at end of InitializeAsync once the session is up; Shutdown transitions back to Unknown + unwires the handler. Unit tests (OpcUaClientSubscribeAndProbeTests, 3 facts): SubscribeAsync_without_initialize_throws_InvalidOperationException, UnsubscribeAsync_with_unknown_handle_is_noop (session-drop-race safety), GetHostStatuses_returns_endpoint_url_row_pre_init (asserts EndpointUrl as the host identity -- the full opc.tcp://plc.example:4840 URL). Live-session subscribe/unsubscribe round-trip + keep-alive state transition coverage lands in a follow-up PR once we scaffold the in-process OPC UA server fixture. 13/13 OpcUaClient.Tests pass. dotnet build clean. All six capability interfaces (IDriver / ITagDiscovery / IReadable / IWritable / ISubscribable / IHostConnectivityProbe) implemented — OPC UA Client driver surface complete.

tests/ZB.MOM.WW.OtOpcUa.Driver.OpcUaClient.Tests/OpcUaClientSubscribeAndProbeTests.cs

@@ -0,0 +1,50 @@
+using Shouldly;
+using Xunit;
+using ZB.MOM.WW.OtOpcUa.Core.Abstractions;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.OpcUaClient.Tests;
+
+/// <summary>
+///     Scaffold tests for <c>ISubscribable</c> + <c>IHostConnectivityProbe</c> that don't
+///     need a live remote server. Live-session tests (subscribe/unsubscribe round-trip,
+///     keep-alive transitions) land in a follow-up PR once the in-process OPC UA server
+///     fixture is scaffolded.
+/// </summary>
+[Trait("Category", "Unit")]
+public sealed class OpcUaClientSubscribeAndProbeTests
+{
+    [Fact]
+    public async Task SubscribeAsync_without_initialize_throws_InvalidOperationException()
+    {
+        using var drv = new OpcUaClientDriver(new OpcUaClientDriverOptions(), "opcua-sub-uninit");
+        await Should.ThrowAsync<InvalidOperationException>(async () =>
+            await drv.SubscribeAsync(["ns=2;s=Demo"], TimeSpan.FromMilliseconds(100), TestContext.Current.CancellationToken));
+    }
+
+    [Fact]
+    public async Task UnsubscribeAsync_with_unknown_handle_is_noop()
+    {
+        using var drv = new OpcUaClientDriver(new OpcUaClientDriverOptions(), "opcua-sub-unknown");
+        // UnsubscribeAsync returns cleanly for handles it doesn't recognise — protects against
+        // the caller's race with server-side cleanup after a session drop.
+        await drv.UnsubscribeAsync(new FakeHandle(), TestContext.Current.CancellationToken);
+    }
+
+    [Fact]
+    public void GetHostStatuses_returns_endpoint_url_row_pre_init()
+    {
+        using var drv = new OpcUaClientDriver(
+            new OpcUaClientDriverOptions { EndpointUrl = "opc.tcp://plc.example:4840" },
+            "opcua-hosts");
+        var rows = drv.GetHostStatuses();
+        rows.Count.ShouldBe(1);
+        rows[0].HostName.ShouldBe("opc.tcp://plc.example:4840",
+            "host identity mirrors the endpoint URL so the Admin /hosts dashboard can link back to the remote server");
+        rows[0].State.ShouldBe(HostState.Unknown);
+    }
+
+    private sealed class FakeHandle : ISubscriptionHandle
+    {
+        public string DiagnosticId => "fake";
+    }
+}