Diagnostics: surface upstream WS close reason + break the null-spin loop

The Pi journal showed 170 000 'upstream silent NxN' messages within one
second — i.e. ReceiveJsonAsync was returning null in zero time, so the
upstream WS to OpenAI was closed. The relay was spinning until idle
watchdog (30 s) fired. Add IRealtimeUpstream.IsClosed + CloseReason,
populated by OpenAIRealtimeUpstream from the close status / WS exception,
and have PumpAsync stop on close and forward
  error code=upstream_closed message=close status=<X> desc=<Y>
to the device. Closes the spin AND gives us the OpenAI close reason.

backend/DeviceHub/DeviceHubEndpoint.cs

@@ -179,7 +179,7 @@ public static class DeviceHubEndpoint
 
     // Bump on every backend change so we can verify a Coolify deploy landed
     // by looking at the hello_ack config.server_version field in the Pi journal.
-    public const string ServerVersion = "plan3-debug-2026-06-12-09";
+    public const string ServerVersion = "plan3-debug-2026-06-12-09-closeprobe";
 
     private static async Task SendHelloAckAsync(ActiveDevice active, Device device, CancellationToken ct)
     {