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/Realtime/IRealtimeUpstream.cs

@@ -6,4 +6,8 @@ public interface IRealtimeUpstream : IAsyncDisposable
 {
     Task SendJsonAsync(JsonObject envelope, CancellationToken ct);
     Task<JsonObject?> ReceiveJsonAsync(CancellationToken ct);
+    // Once a receive returns null because the WebSocket transitioned out of
+    // the Open state, these surface why. Both stay null until that happens.
+    bool IsClosed { get; }
+    string? CloseReason { get; }
 }