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()
     {