fix(r2-01): probe failures mark the handle dead under the gate so the next tick reopens (STAB-15b, task 9)

src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.S7/S7Driver.cs

@@ -1596,17 +1596,39 @@ public sealed class S7Driver
                 await _gate.WaitAsync(probeCts.Token).ConfigureAwait(false);
                 try
                 {
-                    // EnsureConnectedAsync doubles as the reconnect backstop: if a read/write
-                    // marked the handle dead (or the socket dropped between ticks), the probe
-                    // reopens it here so recovery doesn't wait for the next data call.
-                    var plc = await EnsureConnectedAsync(probeCts.Token).ConfigureAwait(false);
-                    await plc.ReadStatusAsync(probeCts.Token).ConfigureAwait(false);
-                    success = true;
+                    try
+                    {
+                        // EnsureConnectedAsync doubles as the reconnect backstop: if a read/write
+                        // marked the handle dead (or the socket dropped between ticks), the probe
+                        // reopens it here so recovery doesn't wait for the next data call.
+                        var plc = await EnsureConnectedAsync(probeCts.Token).ConfigureAwait(false);
+                        await plc.ReadStatusAsync(probeCts.Token).ConfigureAwait(false);
+                        success = true;
+                    }
+                    catch (OperationCanceledException) when (ct.IsCancellationRequested)
+                    {
+                        // Loop teardown — rethrow to the outer filter (after finally releases the gate).
+                        throw;
+                    }
+                    catch (OperationCanceledException)
+                    {
+                        // Probe deadline fired mid-PDU: the CPU-status response may still be in
+                        // flight, so the handle can't be reused (half-read ⇒ desync). We hold _gate
+                        // here, keeping _plcDead gate-guarded (STAB-15b).
+                        _plcDead = true;
+                    }
+                    catch (Exception ex)
+                    {
+                        // STAB-15b: classify + mark while holding _gate so a wire-dead/desynced
+                        // handle whose TcpClient still claims Connected is reopened by the NEXT
+                        // tick's EnsureConnectedAsync instead of being re-probed forever.
+                        MarkConnectionDeadIfFatal(ex);
+                    }
                 }
                 finally { _gate.Release(); }
             }
             catch (OperationCanceledException) when (ct.IsCancellationRequested) { return; }
-            catch { /* transport/timeout/exception — treated as Stopped below */ }
+            catch { /* gate-wait timeout / other — treated as Stopped below (gate contention is not a connection fault) */ }
 
             TransitionTo(success ? HostState.Running : HostState.Stopped);