fix(dcl+centralui): MxGateway tag browse — lazy attributes, frame-size cap, wider scrollable picker

Expanding a Galaxy object in the tag picker hung on "loading…": the browse
reply inlined every child's full attribute set (~152 KB), exceeding Akka's
128 KB remote frame, and remoting silently discarded the oversized reply.

Browse path (DataConnectionLayer):
- RealMxGatewayClient: navigation now uses BrowseChildren(include_attributes=
  false) — child objects only — and an object's own attributes load lazily via
  DiscoverHierarchy(root, max_depth=0) when it's expanded. Payload drops from
  ~152 KB/level to a few KB. Seam contract unchanged.
- DataConnectionActor.CapBrowseChildren: protocol-agnostic byte-budget cap
  (~100 KB) on every BrowseNodeResult before it crosses the site→central
  frame, OR-ing the adapter's own Truncated flag. Byte budget, not a count —
  the only bound that holds regardless of NodeId/attribute-name length.
- RealOpcUaClient: requestedMaxReferencesPerNode 1000 → 500 to narrow the
  window before the byte budget applies.
- Graceful gRPC Unimplemented handling → NotSupportedException →
  BrowseFailureKind.NotBrowsable with an actionable message (older gateway
  builds lacking BrowseChildren).

Picker UI (CentralUI):
- NodeBrowserDialog: modal-lg → modal-xl; new scoped .razor.css caps the tree
  at 55vh with its own scrollbar so manual entry + Select/Cancel stay visible.
- Protocol-agnostic failure messages (was hardcoded "OPC UA …"); renamed the
  leftover opcua-browser-tree class to node-browser-tree.

Tests: new frame-budget cap test + NotSupported=>NotBrowsable mapping test;
DCL suite 88/88. Doc: Component-DataConnectionLayer.md records the lazy
attribute-light browse and the frame-size guard.

src/ZB.MOM.WW.ScadaBridge.DataConnectionLayer/Actors/DataConnectionActor.cs

@@ -989,7 +989,7 @@ public class DataConnectionActor : UntypedActor, IWithStash, IWithTimers
     ///
     /// Failure mapping:
     /// <list type="bullet">
-    ///   <item><see cref="BrowseFailureKind.NotBrowsable"/> — adapter is not <see cref="IBrowsableDataConnection"/>.</item>
+    ///   <item><see cref="BrowseFailureKind.NotBrowsable"/> — adapter is not <see cref="IBrowsableDataConnection"/>, or it threw <see cref="NotSupportedException"/> (browsable adapter, but the server/protocol cannot browse — e.g. a gateway build predating the browse RPC); message carried verbatim in the latter case.</item>
     ///   <item><see cref="BrowseFailureKind.ConnectionNotConnected"/> — adapter threw <see cref="ConnectionNotConnectedException"/>.</item>
     ///   <item><see cref="BrowseFailureKind.Timeout"/> — adapter threw <see cref="OperationCanceledException"/>.</item>
     ///   <item><see cref="BrowseFailureKind.ServerError"/> — any other exception, message carried verbatim.</item>
@@ -1015,13 +1015,16 @@ public class DataConnectionActor : UntypedActor, IWithStash, IWithTimers
             return;
         }
 
-        _log.Debug("[{0}] Browsing OPC UA children of {1}", _connectionName, command.ParentNodeId ?? "(root)");
+        _log.Debug("[{0}] Browsing children of {1}", _connectionName, command.ParentNodeId ?? "(root)");
 
         browsable.BrowseChildrenAsync(command.ParentNodeId).ContinueWith(t =>
         {
             if (t.IsCompletedSuccessfully)
             {
-                return new BrowseNodeResult(t.Result.Children, t.Result.Truncated, Failure: null);
+                // Bound the reply to stay under Akka's remote frame size before it
+                // crosses the site→central boundary (see CapBrowseChildren).
+                var (children, truncated) = CapBrowseChildren(t.Result.Children, t.Result.Truncated);
+                return new BrowseNodeResult(children, truncated, Failure: null);
             }
 
             var baseEx = t.Exception?.GetBaseException();
@@ -1035,6 +1038,13 @@ public class DataConnectionActor : UntypedActor, IWithStash, IWithTimers
                     Array.Empty<BrowseNode>(),
                     Truncated: false,
                     new BrowseFailure(BrowseFailureKind.Timeout, "Browse cancelled.")),
+                // Adapter reachable but the protocol/server cannot browse (e.g. an
+                // MxGateway build that predates the BrowseChildren RPC). Carry the
+                // adapter's explanatory message through as NotBrowsable.
+                NotSupportedException notSupported => new BrowseNodeResult(
+                    Array.Empty<BrowseNode>(),
+                    Truncated: false,
+                    new BrowseFailure(BrowseFailureKind.NotBrowsable, notSupported.Message)),
                 _ => new BrowseNodeResult(
                     Array.Empty<BrowseNode>(),
                     Truncated: false,
@@ -1045,6 +1055,47 @@ public class DataConnectionActor : UntypedActor, IWithStash, IWithTimers
         }).PipeTo(sender);
     }
 
+    /// <summary>
+    /// Estimated-byte ceiling for a single <see cref="BrowseNodeResult"/>, kept
+    /// comfortably below Akka's default 128 KB remote frame size. A browse reply
+    /// crosses the site→central frame on a temp Ask actor; an oversized reply is
+    /// silently discarded by remoting (the picker then hangs on "loading…"). The
+    /// limit is a byte budget rather than a child count because the only thing
+    /// that actually consumes frame space is serialized size — OPC UA NodeIds and
+    /// MxGateway tag references vary widely in length, so a fixed count is not a
+    /// safe proxy.
+    /// </summary>
+    private const int BrowseResultByteBudget = 100 * 1024;
+
+    /// <summary>
+    /// Truncates a browse child list to <see cref="BrowseResultByteBudget"/> using
+    /// a conservative per-node size estimate (JSON structural overhead plus the two
+    /// variable-length strings — ASCII NodeId/DisplayName ≈ 1 byte/char). Returns
+    /// the kept prefix and a <c>Truncated</c> flag OR-ed with the adapter's own
+    /// truncation signal, so the picker shows its "use manual entry" hint when the
+    /// level is clipped. Protocol-agnostic: every adapter's reply funnels through
+    /// here regardless of how it paginates upstream.
+    /// </summary>
+    private static (IReadOnlyList<BrowseNode> Children, bool Truncated) CapBrowseChildren(
+        IReadOnlyList<BrowseNode> children, bool truncated)
+    {
+        var budget = 0;
+        var kept = new List<BrowseNode>(children.Count);
+        foreach (var node in children)
+        {
+            budget += 64 + (node.NodeId?.Length ?? 0) + (node.DisplayName?.Length ?? 0);
+            if (budget > BrowseResultByteBudget)
+            {
+                truncated = true;
+                break;
+            }
+
+            kept.Add(node);
+        }
+
+        return (kept, truncated);
+    }
+
     // ── Test Bindings (one-shot live read of bound tags) ──
 
     /// <summary>