fix(focas): fail-fast at init on unimplemented backend (operator footgun)

Add IFocasClientFactory.EnsureUsable() — a config-time probe called by
FocasDriver.InitializeAsync before any background loops start. The
UnimplementedFocasClientFactory throws NotSupportedException immediately
(faulting the driver at init), eliminating the footgun where a driver on
the 'unimplemented' backend appeared Healthy then failed every read/write/
subscribe silently. WireFocasClientFactory and FakeFocasClientFactory are
no-ops. Backstop Create() throw remains in place.

tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.FOCAS.Tests/FakeFocasClient.cs

@@ -169,6 +169,9 @@ internal sealed class FakeFocasClientFactory : IFocasClientFactory
     /// <summary>Gets or sets a customization function for creating clients.</summary>
     public Func<FakeFocasClient>? Customise { get; set; }
 
+    /// <summary>No-op usability probe — the fake factory is always usable.</summary>
+    public void EnsureUsable() { }
+
     /// <summary>Creates a fake FOCAS client.</summary>
     public IFocasClient Create()
     {

tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.FOCAS.Tests/FocasScaffoldingTests.cs

@@ -261,4 +261,36 @@ public sealed class FocasScaffoldingTests
         ex.Message.ShouldContain("wire");
         ex.Message.ShouldContain("docs/drivers/FOCAS.md");
     }
+
+    /// <summary>
+    ///     Verifies a driver configured with the unimplemented backend faults at
+    ///     InitializeAsync — not lazily on the first read. The operator must get an
+    ///     actionable error immediately rather than a phantom-Healthy driver that
+    ///     fails every read silently.
+    /// </summary>
+    [Fact]
+    public async Task Unimplemented_backend_faults_at_InitializeAsync_not_on_first_read()
+    {
+        var drv = new FocasDriver(
+            new FocasDriverOptions { Devices = [new FocasDeviceOptions("focas://10.0.0.5:8193")] },
+            "drv-unimpl",
+            clientFactory: new UnimplementedFocasClientFactory());
+
+        var ex = await Should.ThrowAsync<NotSupportedException>(
+            () => drv.InitializeAsync("{}", CancellationToken.None));
+
+        ex.Message.ShouldContain("unimplemented");
+        drv.GetHealth().State.ShouldBe(DriverState.Faulted);
+    }
+
+    /// <summary>
+    ///     Verifies WireFocasClientFactory.EnsureUsable is a no-op — the wire backend
+    ///     must still initialize clean (no false-positive faults from the probe).
+    /// </summary>
+    [Fact]
+    public void Wire_factory_EnsureUsable_does_not_throw()
+    {
+        var factory = new Wire.WireFocasClientFactory();
+        Should.NotThrow(() => factory.EnsureUsable());
+    }
 }