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) =>

tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Ab.Transport.Tests/Eip/EncapsulationClientTests.cs

@@ -32,6 +32,16 @@ public sealed class EncapsulationClientTests
         FakeEipServer.Command(server.Received[0]).ShouldBe(EncapCommand.RegisterSession);
     }
 
+    [Fact]
+    public async Task RegisterSession_goes_on_the_wire_with_a_zero_sender_context()
+    {
+        // ab_server refuses RegisterSession with any non-zero sender context (status 1, header-only
+        // reply), so the client must not stamp its per-request context on it. Found by Task 17.
+        await using var server = new FakeEipServer();
+        await using var client = await Connect(server);
+        EncapsulationCodec.ReadHeader(server.Received[0]).SenderContext.ShouldBe(0UL);
+    }
+
     [Fact]
     public async Task SendRRData_returns_cip_payload()
     {

tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Ab.Transport.Tests/Eip/EncapsulationCodecTests.cs

@@ -10,8 +10,8 @@ public sealed class EncapsulationCodecTests
 
     [Fact]
     public void RegisterSession_frame_is_byte_exact() =>
-        Convert.ToHexString(EncapsulationCodec.RegisterSession(Ctx)).ShouldBe(
-            "6500" + "0400" + "00000000" + "00000000" + "0102030405060708" + "00000000" + "0100" + "0000");
+        Convert.ToHexString(EncapsulationCodec.RegisterSession()).ShouldBe(
+            "6500" + "0400" + "00000000" + "00000000" + "0000000000000000" + "00000000" + "0100" + "0000");
 
     [Fact]
     public void UnRegisterSession_frame_is_header_only() =>
@@ -43,13 +43,12 @@ public sealed class EncapsulationCodecTests
             .Status.ShouldBe(AbStatus.TooLarge);
 
     [Fact]
-    public void RegisterSession_matches_libplctag_capture_except_context()
+    public void RegisterSession_matches_libplctag_capture_byte_for_byte()
     {
+        // Including the sender context: ab_server rejects a non-zero RegisterSession context with
+        // encapsulation status 1 (found by Task 17 controllogix — every native session failed).
         var captured = CaptureFile.Load("logix-register-session.hex").First(f => f.FromClient).Bytes;
-        var ours = EncapsulationCodec.RegisterSession(Ctx);
-        ours.Length.ShouldBe(captured.Length);
-        ours.AsSpan(0, 12).ToArray().ShouldBe(captured.AsSpan(0, 12).ToArray());   // cmd,len,session,status
-        ours.AsSpan(20).ToArray().ShouldBe(captured.AsSpan(20).ToArray());         // options + data
+        Convert.ToHexString(EncapsulationCodec.RegisterSession()).ShouldBe(Convert.ToHexString(captured));
     }
 
     [Fact]