fix(centralui): enable Test Bindings for MxGateway connections

The Test Bindings button was disabled (greyed out) for any attribute bound
to a non-OPC-UA connection. BuildTestableRows() filtered to protocol ==
"OpcUa", a stale gate left over from when OPC UA was the only protocol.
ReadTagValuesCommand is protocol-agnostic (routes through
IDataConnection.ReadBatchAsync, which MxGatewayDataConnection implements),
so the filter only blocked the UI — mirroring the already-fixed IsBrowsable.

Remove the OPC-UA-only filter and update the stale comments. Add a bUnit
regression test (theory over MxGateway + OpcUa) asserting the button is
enabled for a readable-protocol binding.

Verified live: dialog opens for an MxGateway binding and returns a
Good-quality read.

src/ZB.MOM.WW.ScadaBridge.CentralUI/Components/Pages/Deployment/InstanceConfigure.razor

@@ -158,9 +158,10 @@
                         <button class="btn btn-success btn-sm" @onclick="SaveBindings" disabled="@_saving">Save Bindings</button>
                         @* Test Bindings: one-shot live read of every bound attribute
                            whose row has a connection picked AND an effective tag
-                           path. Disabled when no testable rows. Currently OPC UA
-                           only — other protocols (none yet) would need their own
-                           wire+adapter support to round-trip through ReadTagValuesCommand. *@
+                           path. Disabled when no testable rows. Protocol-agnostic —
+                           any connection whose adapter implements ReadBatchAsync
+                           (OPC UA and MxGateway today) round-trips through
+                           ReadTagValuesCommand. *@
                         <button class="btn btn-outline-primary btn-sm"
                                 @onclick="OpenTestBindings"
                                 disabled="@(!HasTestableBindings())">
@@ -612,8 +613,10 @@
 
     /// <summary>
     /// Builds the list of testable rows: attributes that have a connection
-    /// picked AND a non-empty effective tag path AND an OPC UA connection
-    /// (the only protocol routed through <c>ReadTagValuesCommand</c> today).
+    /// picked AND a non-empty effective tag path. Protocol-agnostic — every
+    /// data-connection adapter implements <c>ReadBatchAsync</c>, so the read
+    /// routes through <c>ReadTagValuesCommand</c> regardless of protocol
+    /// (OPC UA and MxGateway today).
     /// </summary>
     private List<TestBindingsDialog.BindingRowToTest> BuildTestableRows()
     {
@@ -626,10 +629,11 @@
             var conn = _siteConnections.FirstOrDefault(c => c.Id == connId);
             if (conn is null) continue;
 
-            // OPC UA only — other protocols don't have a site-side
-            // ReadTagValuesCommand handler wired up yet.
-            if (!string.Equals(conn.Protocol, "OpcUa", StringComparison.OrdinalIgnoreCase))
-                continue;
+            // Protocol-agnostic: ReadTagValuesCommand routes through the
+            // site-side IDataConnection.ReadBatchAsync contract, which every
+            // adapter implements (OPC UA and MxGateway today). A not-connected
+            // or unsupported connection short-circuits to a typed banner in the
+            // dialog rather than being filtered out here — mirrors IsBrowsable.
 
             var effectivePath = _bindingOverrides.GetValueOrDefault(attr.Name)
                                 ?? GetTemplateDefault(attr.Name);