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.Server/Metrics/GatewayMetrics.cs

@@ -78,6 +78,15 @@ public sealed class GatewayMetrics : IDisposable
         _meter.CreateObservableGauge("mxgateway.events.grpc_stream_queue.depth", GetGrpcEventStreamQueueDepth);
     }
 
+    /// <summary>
+    /// Gets the underlying <see cref="Meter"/> instance backing this metrics object. Exposed to tests
+    /// (via <c>InternalsVisibleTo</c>) so a <see cref="MeterListener"/> can filter measurements by
+    /// <see cref="object.ReferenceEquals"/> against this specific instance rather than by the
+    /// process-shared <see cref="MeterName"/>, which would cross-talk between parallel tests that
+    /// each build their own <see cref="GatewayMetrics"/> (Tests-027).
+    /// </summary>
+    internal Meter Meter => _meter;
+
     /// <summary>
     /// Records that a session has been opened.
     /// </summary>