Fix flaky data channel budget assertions

Check per-flow DATA reservations instead of the shared message total, which also counts concurrently admitted control frames. Dispose shared Core contexts before asserting that close-control reservations have drained after PubSub stops.

tests/Opc.Ua.Core.DataChannels.Tests/Runtime/FlowSendAdmissionTests.cs

@@ -181,7 +181,8 @@ namespace Opc.Ua.Core.DataChannels.Tests.Runtime
                 {
                     Assert.That(pending.IsCompleted, Is.False);
                     Assert.That(waiting.Local.StateData.LastDataId, Is.Zero);
-                    Assert.That(budget.Usage.Messages, Is.EqualTo(heldCount));
+                    Assert.That(occupied.Local.StateData.SendMessages, Is.EqualTo(heldCount));
+                    Assert.That(waiting.Local.StateData.SendMessages, Is.Zero);
                     first.InitiatorCarrier.BeforeRelease.TrySetResult(true);
                     Assert.That(await Task.WhenAll(held).WaitAsync(TimeSpan.FromSeconds(5)).ConfigureAwait(false),
                         Is.All.EqualTo(DataChannelResult.Good));

tests/Opc.Ua.PubSub.Tests/Transports/DataChannels/DataChannelPubSubRuntimeTests.cs

@@ -431,6 +431,12 @@ namespace Opc.Ua.PubSub.Tests.Transports.DataChannels
             {
                 Assert.That(core.LeftBudget.Usage.Flows, Is.Zero);
                 Assert.That(core.RightBudget.Usage.Flows, Is.Zero);
+            });
+            // Shared Core contexts can still own close-control reservations after the PubSub flows stop.
+            await core.Left.DisposeAsync().ConfigureAwait(false);
+            await core.Right.DisposeAsync().ConfigureAwait(false);
+            Assert.Multiple(() =>
+            {
                 Assert.That(core.LeftBudget.Usage.Messages, Is.Zero);
                 Assert.That(core.RightBudget.Usage.Messages, Is.Zero);
             });