Resolve Tests-027..031: flake root cause + coverage gaps

Tests-027  GatewayMetrics exposes its internal Meter; the
           StreamEvents_WhenEventIsWritten_RecordsSendDuration listener
           now filters by ReferenceEquals(instrument.Meter, metrics.Meter)
           instead of Meter.Name, so parallel tests with their own
           GatewayMetrics no longer cross-contaminate the families list.
Tests-028  FakeWorkerClient.Kill now captures LastKillReason;
           SessionManager.KillWorkerAsync tests pin the reason
           propagation end-to-end and cover the blank/null guard. The
           DashboardSessionAdminService kill test pins the literal
           dashboard-admin-kill reason.
Tests-029  Added CloseSessionAsync_BlankSessionId_ReturnsFailure to mirror
           the existing KillWorkerAsync blank-id coverage.
Tests-030  DeleteAsync_WhenStoreRefuses_ReportsFriendlyError renamed and
           extended to assert the dashboard-delete-key audit row with
           Details = not-found-or-active. Added
           DeleteAsync_BlankKeyId_ReturnsFailure.
Tests-031  DashboardSnapshotPublisher reconnect test now measures the
           gap from the first throw inside the fake (firstThrowAt) to
           secondSubscribeAt, isolating Task.Delay from StartAsync /
           scheduling overhead.

All resolved at 2026-05-24; 512/512 gateway tests pass.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/ZB.MOM.WW.MxGateway.Tests/Gateway/Dashboard/DashboardSessionAdminServiceTests.cs

@@ -87,7 +87,12 @@ public sealed class DashboardSessionAdminServiceTests
         Assert.True(result.Succeeded);
         Assert.Equal(1, sessionManager.KillCount);
         Assert.Equal("session-1", sessionManager.LastKilledSessionId);
-        Assert.False(string.IsNullOrWhiteSpace(sessionManager.LastKillReason));
+
+        // Tests-028: pin the literal reason string so a future caller-side change is a deliberate
+        // test update rather than a silent drift. DashboardSessionAdminService passes a hard-coded
+        // "dashboard-admin-kill" so the worker-exit metric (mxgateway.workers.killed) carries a
+        // stable, machine-greppable reason tag.
+        Assert.Equal("dashboard-admin-kill", sessionManager.LastKillReason);
     }
 
     [Fact]
@@ -105,6 +110,26 @@ public sealed class DashboardSessionAdminServiceTests
         Assert.Equal(0, sessionManager.KillCount);
     }
 
+    /// <summary>
+    /// Tests-029: <c>CloseSessionAsync</c> has the same blank-session-id guard as
+    /// <c>KillWorkerAsync</c> but previously had no parallel test. Coverage was asymmetric.
+    /// A guard-removal regression on the close path would slip through.
+    /// </summary>
+    [Fact]
+    public async Task CloseSessionAsync_BlankSessionId_ReturnsFailure()
+    {
+        FakeSessionManager sessionManager = new();
+        DashboardSessionAdminService service = CreateService(sessionManager);
+
+        DashboardSessionAdminResult result = await service.CloseSessionAsync(
+            CreateUser(DashboardRoles.Admin),
+            "   ",
+            CancellationToken.None);
+
+        Assert.False(result.Succeeded);
+        Assert.Equal(0, sessionManager.CloseCount);
+    }
+
     [Fact]
     public void CanManage_RejectsUnauthenticatedAndViewer()
     {