test(ab-transport): blocked-write timeout fault path via a stream seam (#565, #593)

Claude-Session: https://claude.ai/code/session_01Ef2ozxS7QPanGT8uoATbTe

src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Ab.Transport/Eip/EncapsulationClient.cs

@@ -39,7 +39,7 @@ public sealed class EncapsulationClient : IAsyncDisposable
     private static readonly TimeSpan UnregisterBudget = TimeSpan.FromMilliseconds(500);
 
     private readonly TcpClient _tcp;
-    private readonly NetworkStream _stream;
+    private readonly Stream _stream;
 
     // Never disposed: an abandoned exchange may release it after DisposeAsync returns, and
     // SemaphoreSlim.Release on a disposed instance throws. It holds no unmanaged resource unless
@@ -56,12 +56,17 @@ public sealed class EncapsulationClient : IAsyncDisposable
     /// <summary>False once the connection has faulted or been disposed.</summary>
     public bool IsConnected => !_faulted && Volatile.Read(ref _disposed) == 0 && _tcp.Connected;
 
-    private EncapsulationClient(TcpClient tcp)
+    private EncapsulationClient(TcpClient tcp, Stream? stream = null)
     {
         _tcp = tcp;
-        _stream = tcp.GetStream();
+        _stream = stream ?? tcp.GetStream();
     }
 
+    /// <summary>Test seam: wraps an already-connected <paramref name="tcp"/> but routes all frame I/O through
+    /// <paramref name="stream"/> (e.g. one whose write never completes), skipping RegisterSession. The TCP
+    /// client is still what <see cref="Fault"/> closes and <see cref="IsConnected"/> consults.</summary>
+    internal static EncapsulationClient ForStream(TcpClient tcp, Stream stream) => new(tcp, stream);
+
     /// <summary>Open the TCP connection and register an encapsulation session. <paramref name="timeout"/>
     /// bounds the TCP connect and the RegisterSession exchange separately.</summary>
     public static async Task<EncapsulationClient> ConnectAsync(string host, int port, TimeSpan timeout, CancellationToken ct)