fix(ab-transport): RegisterSession carries a zero sender context (found by Task 17 controllogix)

ab_server's RegisterSession sanity check rejects any non-zero sender context
with encapsulation status 1 and a header-only reply, so every native session
against the simulator failed with BadCommunicationError. libplctag sends zero
(logix-register-session.hex); the codec test now matches that capture byte for
byte instead of excluding the context, and a client test pins the zero on the wire.

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

@@ -82,7 +82,7 @@ public sealed class EncapsulationClient : IAsyncDisposable
         try
         {
             var (header, _) = await client.ExchangeAsync(connected: false,
-                (ctx, _) => EncapsulationCodec.RegisterSession(ctx), timeout, ct).ConfigureAwait(false);
+                (_, _) => EncapsulationCodec.RegisterSession(), timeout, ct).ConfigureAwait(false);
             client.SessionHandle = header.SessionHandle;
             return client;
         }

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

@@ -30,9 +30,13 @@ public static class EncapsulationCodec
         return w.ToArray();
     }
 
-    /// <summary>RegisterSession: protocol version 1, option flags 0.</summary>
-    public static byte[] RegisterSession(ulong ctx) =>
-        Finish(Header(EncapCommand.RegisterSession, 0, ctx).U16(1).U16(0));
+    /// <summary>RegisterSession: protocol version 1, option flags 0, and a <b>zero</b> sender context.</summary>
+    /// <remarks>The sender context is always zero, as libplctag sends it (<c>logix-register-session.hex</c>):
+    /// ab_server's RegisterSession sanity check rejects any non-zero context with encapsulation status 1
+    /// and a header-only reply, so a per-request context here failed every native session against the
+    /// simulator. The reply is still matched — unconnected matching tolerates a zero context.</remarks>
+    public static byte[] RegisterSession() =>
+        Finish(Header(EncapCommand.RegisterSession, 0, 0).U16(1).U16(0));
 
     /// <summary>UnRegisterSession: header only, no data.</summary>
     public static byte[] UnRegisterSession(uint session, ulong ctx) =>