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.tests/ScriptedRealtimeUpstream.cs

@@ -13,6 +13,8 @@ public class ScriptedRealtimeUpstream : IRealtimeUpstream
         Channel.CreateUnbounded<JsonObject>(new UnboundedChannelOptions { SingleWriter = true });
 
     public IList<JsonObject> Sent { get; } = new List<JsonObject>();
+    public bool IsClosed => _toRelay.Reader.Completion.IsCompleted;
+    public string? CloseReason => IsClosed ? "scripted upstream closed" : null;
 
     public Task SendJsonAsync(JsonObject envelope, CancellationToken ct)
     {