feat!: probes connect to the backend directly

A probe now links to the backend like an agent (probe.json carries the
server address). The backend sends the agent list as desired state and the
probe reports its platform and build in its own hello. The agent no longer
relays the list or the probe platform. Existing probes must be re-created.

backend/tests/Monitor.TestSupport/ProbeSimulator.cs

@@ -61,8 +61,6 @@ public sealed class ProbeSimulator : IAsyncDisposable
 
     public ConcurrentQueue<TestId> Aborts { get; } = new();
 
-    public ConcurrentQueue<ProbeAgentsResult> AgentLists { get; } = new();
-
     public bool IsConnected => _rpc is not null && !_stop.IsCancellationRequested;
 
     public long NowUs() => (TimeProvider.System.GetTimestamp() - _origin) * 1_000_000 / TimeProvider.System.TimestampFrequency;
@@ -347,14 +345,6 @@ public sealed class ProbeSimulator : IAsyncDisposable
                     probe.Aborts.Enqueue(abort.TestId);
                     return ValueTask.CompletedTask;
                 })
-            .OnNotify(
-                MonitorMethods.ProbeAgentsChanged,
-                MonitorJsonContext.Default.ProbeAgentsResult,
-                (probe, list, _) =>
-                {
-                    probe.AgentLists.Enqueue(list);
-                    return ValueTask.CompletedTask;
-                })
             .OnNotify(
                 FleetMethods.OutboxAck,
                 FleetRpcContext.Default.OutboxAckParams,