fix(focas): a caller's own cancellation during connect does not open the backoff window; deterministic concurrent-connect test

Review follow-up on c940349f: a cancelled caller recorded a connect failure, throttling every caller queued
on ConnectGate for ~1 s. The 200 ms settle delay in the concurrent-connect test is replaced by a
TaskCompletionSource the fake connect signals on entry. Both tests verified red with their fix reverted.

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

src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.FOCAS/FocasDriver.cs

@@ -1855,12 +1855,15 @@ public sealed class FocasDriver : IDriver, IReadable, IWritable, ITagDiscovery,
             {
                 await client.ConnectAsync(device.ParsedAddress, _options.Timeout, ct).ConfigureAwait(false);
             }
-            catch
+            catch (Exception ex)
             {
                 // Dispose only OUR client — never one another path installed meanwhile (recycle/teardown).
                 client.Dispose();
                 if (ReferenceEquals(device.Client, client)) device.Client = null;
-                device.Backoff.RecordFailure(DateTime.UtcNow); // 05/STAB-8: open the backoff window
+                // The caller's OWN cancellation says nothing about the CNC: do not open the backoff window, or
+                // every caller queued on ConnectGate behind it would be throttled for ~1 s for nothing.
+                if (!(ex is OperationCanceledException && ct.IsCancellationRequested))
+                    device.Backoff.RecordFailure(DateTime.UtcNow); // 05/STAB-8: open the backoff window
                 throw;
             }
             device.Backoff.RecordSuccess(); // 05/STAB-8: recovery resets the window immediately