mbproxy: close out the dashboard code-review minor findings

Resolves the remaining Minor items from the 2026-05-15 review so the
web-UI dashboard work has no open follow-ups: a real-HubConnection
end-to-end test for the SignalR feed, stable mbproxy.admin.broadcast.*
log-event names, keyboard/aria accessibility on the fleet table,
frontend JS hardening (URL-decode guard, NaN guards, shared util.js),
reconciler<->capture-registry coverage, throwing-sink and embedded-asset
tests, broadcaster polish, and a soft upper bound on AdminPushIntervalMs.

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

mbproxy/tests/Mbproxy.Tests/Admin/SignalRFakes.cs

@@ -64,3 +64,18 @@ internal sealed class FakeStatusPushSink : IStatusPushSink
         return Task.CompletedTask;
     }
 }
+
+/// <summary>
+/// An <see cref="IStatusPushSink"/> whose every push fails with an exception from the
+/// supplied factory — used to prove <see cref="StatusBroadcaster.PushOnceAsync"/> swallows
+/// a transport fault (and that its <c>when (ex is not OperationCanceledException)</c> filter
+/// still lets a cancellation propagate).
+/// </summary>
+internal sealed class ThrowingStatusPushSink(Func<Exception> exceptionFactory) : IStatusPushSink
+{
+    public Task PushFleetAsync(StatusResponse snapshot, CancellationToken ct)
+        => throw exceptionFactory();
+
+    public Task PushPlcAsync(string plcName, PlcDetailResponse detail, CancellationToken ct)
+        => throw exceptionFactory();
+}