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.

src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.FOCAS/FocasDriver.cs

@@ -89,6 +89,11 @@ public sealed class FocasDriver : IDriver, IReadable, IWritable, ITagDiscovery,
         Volatile.Write(ref _health, new DriverHealth(DriverState.Initializing, null, null));
         try
         {
+            // Fail fast if the factory is a stub/unimplemented backend — the operator must
+            // see an actionable error at init rather than a phantom-Healthy driver that fails
+            // every read/write/subscribe silently (Driver.FOCAS-009).
+            _clientFactory.EnsureUsable();
+
             foreach (var device in _options.Devices)
             {
                 var addr = FocasHostAddress.TryParse(device.HostAddress)