Add runtime address space rebuild integration tests

Tests verify nodes can be added/removed from the OPC UA server at
runtime by mutating FakeGalaxyRepository and triggering a rebuild.
Uses real OPC UA client sessions to browse, subscribe, and observe
changes.

Tests cover:
- Browse initial hierarchy via OPC UA client
- Add object at runtime → new node appears on browse
- Remove object → node disappears from browse
- Subscribe to node, then remove it → publishes Bad quality
- Surviving nodes still browsable after partial rebuild
- Add/remove individual attributes at runtime

Infrastructure:
- OpcUaTestClient helper for programmatic OPC UA client connections
- OpcUaServerFixture updated with GalaxyRepository/MxProxy accessors
- OpcUaService.TriggerRebuild() exposed for test-driven rebuilds
- Namespace index resolved dynamically via session namespace table

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/ZB.MOM.WW.LmxOpcUa.Host/OpcUaService.cs

@@ -267,6 +267,11 @@ namespace ZB.MOM.WW.LmxOpcUa.Host
             Log.Fatal(e.ExceptionObject as Exception, "Unhandled exception (IsTerminating={IsTerminating})", e.IsTerminating);
         }
 
+        /// <summary>
+        /// Triggers an address space rebuild from the current Galaxy repository data. For testing.
+        /// </summary>
+        internal void TriggerRebuild() => OnGalaxyChanged();
+
         // Accessors for testing
         internal IMxAccessClient? MxClient => (IMxAccessClient?)_mxAccessClient ?? _mxAccessClientForWiring;
         internal PerformanceMetrics? Metrics => _metrics;