fix(opcuaclient): shutdown cancels session work before the gate wait; dispose skips a redundant nested shutdown (review r2 R2-1, R2-2)

- R2-1: the ConditionRefresh sweeps (RefreshRequired, post-outage recovery) run
  under the session's work token, and ShutdownAsync cancels session work first
  thing - so a sweep holding the subscription gate against a stalling upstream no
  longer eats the whole teardown budget the deletes and close need.
- R2-2: DisposeAsync skips its nested ShutdownAsync when a shutdown already started
  for the current lifecycle generation (nothing initialised since), so PostStop's
  dispose does not re-wait the gate past its budget. Test: with the gate held, the
  shutdown waits (positive control) and the dispose does not wait again; red with
  the skip removed.

Refs #743

Claude-Session: https://claude.ai/code/session_01Ub9hGmYLQvn9XETzw7CMsT

tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.OpcUaClient.Tests/OpcUaClientLifecycleGenerationTests.cs

@@ -257,4 +257,25 @@ public sealed class OpcUaClientLifecycleGenerationTests
         session.Verify(s => s.AddSubscription(It.IsAny<Subscription>()), Times.Once,
             "the late subscribe must not have created anything on the closing session");
     }
+
+    /// <summary>
+    ///     r2 R2-2: PostStop runs ShutdownAsync then DisposeAsync. With subscription management contended (a subscribe
+    ///     still holding the gate), the shutdown spends its budget waiting on the gate; the dispose that follows must not
+    ///     re-run a nested shutdown and wait all over again. Positive control: the shutdown itself did wait.
+    /// </summary>
+    [Fact]
+    public async Task Dispose_after_a_shutdown_of_the_same_lifecycle_does_not_rewait_the_gate()
+    {
+        var drv = new OpcUaClientDriver(Options(), "opcua-r22") { TeardownBudget = TimeSpan.FromSeconds(3) };
+        drv.SetSessionForTest(NewSession().Object);
+        await drv.SubscriptionGate.WaitAsync(TestContext.Current.CancellationToken); // a "subscribe" that never finishes
+
+        var sw = Stopwatch.StartNew();
+        await drv.ShutdownAsync(TestContext.Current.CancellationToken);
+        sw.Elapsed.ShouldBeGreaterThan(TimeSpan.FromSeconds(2), "positive control: the shutdown waited on the held gate");
+
+        sw.Restart();
+        await drv.DisposeAsync();
+        sw.Elapsed.ShouldBeLessThan(TimeSpan.FromSeconds(2), "the dispose must not wait the 3 s gate budget a second time");
+    }
 }