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/Wire/WireFocasClient.cs

@@ -427,6 +427,12 @@ public sealed class WireFocasClientFactory : IFocasClientFactory
         _logger = logger;
     }
 
+    /// <summary>
+    ///     No-op usability probe — the wire backend is always usable at config time.
+    ///     Implements <see cref="IFocasClientFactory.EnsureUsable"/>.
+    /// </summary>
+    public void EnsureUsable() { }
+
     /// <summary>Creates a new WireFocasClient instance.</summary>
     /// <returns>A new IFocasClient implementation.</returns>
     public IFocasClient Create() => new WireFocasClient(_logger);