feat(ab-transport): process-wide open-attempt budget so the differential never retries an open (Task 18 review r1)

src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Ab.Transport/Cip/CipConnection.cs

@@ -26,6 +26,18 @@ public sealed class CipConnection : ICipChannel, IAsyncDisposable
     /// <summary>Channel sends by every connection in the process (keep-alives included). Gate diagnostics.</summary>
     internal static int RequestsSent;
 
+    /// <summary>Open attempts (TCP connect + RegisterSession + Forward Open, a fallback open included) started by
+    /// every connection in the process — counted before any I/O, so a refused attempt counts too. Gate diagnostics.</summary>
+    internal static int OpenAttempts;
+
+    /// <summary>
+    ///     Process-wide cap on <see cref="OpenAttempts"/>; <c>-1</c> (the default) is unlimited. An attempt beyond
+    ///     the cap fails with <see cref="AbStatus.Abort"/> before any I/O — no TCP connect, no Forward Open. Only the
+    ///     read-only differential harness (Task 18/21) sets it, so a production press that refuses or drops the
+    ///     connection sees exactly one open attempt instead of one per remaining operation.
+    /// </summary>
+    internal static int OpenAttemptBudget = -1;
+
     /// <summary>Get Attribute Single, Identity (class 1) instance 1 attribute 1 (Vendor ID).</summary>
     internal static readonly byte[] KeepAliveRequest = [0x0E, 0x03, 0x20, 0x01, 0x24, 0x01, 0x30, 0x01];
 
@@ -176,6 +188,11 @@ public sealed class CipConnection : ICipChannel, IAsyncDisposable
     private async Task<Open> OpenAsync(CancellationToken ct)
     {
         ThrowIfDisposed();   // disposal may have begun while this attempt waited out the throttle
+        var attempt = Interlocked.Increment(ref OpenAttempts);
+        var budget = Volatile.Read(ref OpenAttemptBudget);
+        if (budget >= 0 && attempt > budget)
+            throw new AbTransportException(AbStatus.Abort,
+                $"Open attempt {attempt} to {_key.Host}:{_key.Port} refused before any I/O: the process-wide open-attempt budget of {budget} is spent.");
         var client = await EncapsulationClient.ConnectAsync(_key.Host, _key.Port, _o.Timeout, ct).ConfigureAwait(false);
         if (Volatile.Read(ref _disposed) != 0)
         {