test: deterministic CI-flake fixes (FOCAS alarm resubscribe, Modbus dead-port connect, S7 array poll)

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

tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.S7.Tests/S7PollEngineMigrationTests.cs

@@ -21,6 +21,9 @@ public sealed class S7PollEngineMigrationTests
     private sealed class ConstantArrayFactory : IS7PlcFactory
     {
         public bool ReadThrows { get; set; }
+        private int _reads;
+        public int Reads => Volatile.Read(ref _reads);
+        public void CountRead() => Interlocked.Increment(ref _reads);
         public IS7Plc Create(S7CpuType cpuType, string host, int port, short rack, short slot, TimeSpan timeout)
             => new ConstantArrayPlc(this);
     }
@@ -39,6 +42,7 @@ public sealed class S7PollEngineMigrationTests
 
         public Task<byte[]> ReadBytesAsync(S7NetDataType area, int db, int startByteAdr, int count, CancellationToken ct)
         {
+            owner.CountRead();
             if (owner.ReadThrows) throw new PlcException(ErrorCode.ConnectionError, "poll failure");
             // Fresh array instance each call, identical contents: BE words 1, 2, 3.
             var block = new byte[count];
@@ -81,7 +85,16 @@ public sealed class S7PollEngineMigrationTests
         drv.OnDataChange += (_, e) => events.Enqueue(e);
 
         var h = await drv.SubscribeAsync(["Arr"], TimeSpan.FromMilliseconds(100), TestContext.Current.CancellationToken);
-        await Task.Delay(500, TestContext.Current.CancellationToken); // several poll cycles
+        // Wait for the initial push (generous deadline: a loaded CI box can starve the first poll), then
+        // for several MORE reads to complete as the positive control that the absence below is meaningful.
+        var deadline = DateTime.UtcNow + TimeSpan.FromSeconds(30);
+        while (events.IsEmpty && DateTime.UtcNow < deadline)
+            await Task.Delay(10, TestContext.Current.CancellationToken);
+        events.Count.ShouldBeGreaterThan(0, "the initial-data push never landed");
+        var readsAtPush = factory.Reads;
+        while (factory.Reads < readsAtPush + 4 && DateTime.UtcNow < deadline)
+            await Task.Delay(10, TestContext.Current.CancellationToken);
+        factory.Reads.ShouldBeGreaterThanOrEqualTo(readsAtPush + 4, "later poll cycles never ran");
         await drv.UnsubscribeAsync(h, TestContext.Current.CancellationToken);
 
         events.Count.ShouldBe(1); // only the initial-data push