feat(otopcua): set Modbus/S7/Galaxy re-discovery policy to Once + Once-branch test (follow-up B)

src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Modbus/ModbusDriver.cs

@@ -261,6 +261,13 @@ public sealed class ModbusDriver
 
     // ---- ITagDiscovery ----
 
+    /// <summary>
+    ///     Run-once: <see cref="DiscoverAsync"/> emits the complete node set synchronously from
+    ///     the configured tag table in a single pass — nothing fills in asynchronously after
+    ///     connect, so a single discovery pass is sufficient.
+    /// </summary>
+    public DiscoveryRediscoverPolicy RediscoverPolicy => DiscoveryRediscoverPolicy.Once;
+
     /// <summary>Discovers tags and builds the OPC UA address space.</summary>
     /// <param name="builder">Address space builder.</param>
     /// <param name="cancellationToken">Cancellation token.</param>