fix(communication): resolve Communication-012..015 — endpoint-aware gRPC client cache, address-change recreation, correlation-id validation, node-flip tests

src/ScadaLink.Communication/Grpc/SiteStreamGrpcClient.cs

@@ -20,6 +20,14 @@ public class SiteStreamGrpcClient : IAsyncDisposable, IDisposable
     private readonly ILogger? _logger;
     private readonly ConcurrentDictionary<string, CancellationTokenSource> _subscriptions = new();
 
+    /// <summary>
+    /// The gRPC endpoint (site node address) this client is bound to. The
+    /// <see cref="SiteStreamGrpcClientFactory"/> compares this against the requested
+    /// endpoint so a NodeA→NodeB failover flip (or a site address edit) is honoured
+    /// rather than served stale from cache.
+    /// </summary>
+    public virtual string Endpoint { get; } = string.Empty;
+
     /// <summary>
     /// The HTTP/2 keepalive ping delay actually applied to this client's channel.
     /// Exposed for tests verifying that <see cref="CommunicationOptions"/> is honoured.
@@ -44,6 +52,7 @@ public class SiteStreamGrpcClient : IAsyncDisposable, IDisposable
     /// </summary>
     public SiteStreamGrpcClient(string endpoint, ILogger logger, CommunicationOptions options)
     {
+        Endpoint = endpoint;
         KeepAlivePingDelay = options.GrpcKeepAlivePingDelay;
         KeepAlivePingTimeout = options.GrpcKeepAlivePingTimeout;
         _channel = GrpcChannel.ForAddress(endpoint, new GrpcChannelOptions
@@ -67,6 +76,16 @@ public class SiteStreamGrpcClient : IAsyncDisposable, IDisposable
     {
     }
 
+    /// <summary>
+    /// Protected constructor for unit testing — records the endpoint without
+    /// opening a real gRPC channel, so endpoint-aware factory behaviour can be
+    /// exercised by test doubles.
+    /// </summary>
+    protected SiteStreamGrpcClient(string endpoint)
+    {
+        Endpoint = endpoint;
+    }
+
     /// <summary>
     /// Creates a test-only instance that has no gRPC channel. Used to test
     /// Unsubscribe and Dispose behavior without needing a real endpoint.