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>

tests/ZB.MOM.WW.LmxOpcUa.Tests/Helpers/OpcUaServerFixture.cs

@@ -25,15 +25,37 @@ namespace ZB.MOM.WW.LmxOpcUa.Tests.Helpers
         public int OpcUaPort { get; }
         public string EndpointUrl => $"opc.tcp://localhost:{OpcUaPort}/LmxOpcUa";
 
+        /// <summary>
+        /// The fake Galaxy repository injected into the service. Mutate Hierarchy/Attributes
+        /// then call Service.TriggerRebuild() to simulate a Galaxy redeployment.
+        /// </summary>
+        public FakeGalaxyRepository? GalaxyRepository { get; }
+
+        /// <summary>
+        /// The fake MxAccess client injected into the service (when using WithFakeMxAccessClient).
+        /// </summary>
+        public FakeMxAccessClient? MxAccessClient { get; }
+
+        /// <summary>
+        /// The fake MxProxy injected into the service (when using WithFakes).
+        /// </summary>
+        public FakeMxProxy? MxProxy { get; }
+
         private readonly OpcUaServiceBuilder _builder;
         private bool _started;
 
-        public OpcUaServerFixture(OpcUaServiceBuilder builder)
+        private OpcUaServerFixture(OpcUaServiceBuilder builder,
+            FakeGalaxyRepository? repo = null,
+            FakeMxAccessClient? mxClient = null,
+            FakeMxProxy? mxProxy = null)
         {
             OpcUaPort = Interlocked.Increment(ref _nextPort);
             _builder = builder;
             _builder.WithOpcUaPort(OpcUaPort);
             _builder.DisableDashboard();
+            GalaxyRepository = repo;
+            MxAccessClient = mxClient;
+            MxProxy = mxProxy;
         }
 
         /// <summary>
@@ -56,7 +78,7 @@ namespace ZB.MOM.WW.LmxOpcUa.Tests.Helpers
                 .WithGalaxyRepository(r)
                 .WithGalaxyName("TestGalaxy");
 
-            return new OpcUaServerFixture(builder);
+            return new OpcUaServerFixture(builder, repo: r, mxProxy: p);
         }
 
         /// <summary>
@@ -79,7 +101,7 @@ namespace ZB.MOM.WW.LmxOpcUa.Tests.Helpers
                 .WithGalaxyRepository(r)
                 .WithGalaxyName("TestGalaxy");
 
-            return new OpcUaServerFixture(builder);
+            return new OpcUaServerFixture(builder, repo: r, mxClient: client);
         }
 
         public Task InitializeAsync()