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/IFocasClient.cs

@@ -282,6 +282,18 @@ public interface IFocasClientFactory
     /// <summary>Creates a new FOCAS client instance.</summary>
     /// <returns>A new IFocasClient instance.</returns>
     IFocasClient Create();
+
+    /// <summary>
+    ///     Config-time usability probe called by <see cref="FocasDriver.InitializeAsync"/> before
+    ///     any background loops start. Must NOT create a live wire client. Implementations that
+    ///     are permanently non-functional (e.g. <see cref="UnimplementedFocasClientFactory"/>)
+    ///     throw <see cref="NotSupportedException"/> here so the driver faults immediately at
+    ///     init rather than lazily on the first read.
+    /// </summary>
+    /// <exception cref="NotSupportedException">
+    ///     Thrown by stub/unimplemented backends to force a fail-fast init fault.
+    /// </exception>
+    void EnsureUsable();
 }
 
 /// <summary>
@@ -292,12 +304,22 @@ public interface IFocasClientFactory
 /// </summary>
 public sealed class UnimplementedFocasClientFactory : IFocasClientFactory
 {
+    private const string Message =
+        "FOCAS driver backend is 'unimplemented'. Switch to 'Backend: \"wire\"' in driver config " +
+        "once the CNC is provisioned — see docs/drivers/FOCAS.md.";
+
+    /// <summary>
+    ///     Config-time probe — throws immediately so <see cref="FocasDriver.InitializeAsync"/>
+    ///     faults the driver before any background loops start. This prevents the footgun where
+    ///     the driver appears Healthy at init but every read/write/subscribe fails.
+    /// </summary>
+    /// <exception cref="NotSupportedException">Always thrown.</exception>
+    public void EnsureUsable() => throw new NotSupportedException(Message);
+
     /// <summary>Creates a new client instance (always throws NotSupportedException).</summary>
     /// <returns>Never returns; always throws NotSupportedException.</returns>
     /// <exception cref="NotSupportedException">Always thrown to indicate backend is not yet provisioned.</exception>
-    public IFocasClient Create() => throw new NotSupportedException(
-        "FOCAS driver backend is 'unimplemented'. Switch to 'Backend: \"wire\"' in driver config " +
-        "once the CNC is provisioned — see docs/drivers/FOCAS.md.");
+    public IFocasClient Create() => throw new NotSupportedException(Message);
 }
 
 /// <summary>