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/Mbproxy.Tests.csproj

@@ -23,6 +23,9 @@
     <PackageReference Include="Shouldly" Version="4.3.0" />
     <!-- NModbus: Modbus TCP client for simulator smoke tests and e2e tests. -->
     <PackageReference Include="NModbus" Version="3.0.83" />
+    <!-- SignalR .NET client — drives the /hub/status end-to-end test (a real
+         HubConnection against the live Kestrel admin host). -->
+    <PackageReference Include="Microsoft.AspNetCore.SignalR.Client" Version="10.0.0" />
   </ItemGroup>
 
   <ItemGroup>