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.Backend.Tests/OperationsAndSecurityTests.cs

@@ -175,9 +175,8 @@ public sealed class OperationsAndSecurityTests
             Assert.Equal("fleet_unsupported", reply.Error!.Data!.Code);
         }
 
-        // 连接仍然可用
-        ProbeAgentsResult list = await agent.RequestProbeAgentsAsync();
-        Assert.Contains(list.Agents, a => a.AgentId == agent.NodeId);
+        // 连接仍然可用：路由器照常回复
+        Assert.Equal("fleet_unsupported", (await agent.RequestRawAsync("fleet.exec")).Error!.Data!.Code);
     }
 
     [Fact]
@@ -195,9 +194,6 @@ public sealed class OperationsAndSecurityTests
     public async Task PendingNodes_CannotCallProductMethods()
     {
         await using AgentSimulator agent = await _backend.ConnectAgentAsync(autoApprove: false);
-        RpcMessage reply = await agent.RequestRawAsync(MonitorMethods.ProbeAgents, JsonSerializer.SerializeToElement(new ProbeAgentsParams(), MonitorJsonContext.Default.ProbeAgentsParams));
-        Assert.Equal("fleet_node_pending", reply.Error!.Data!.Code);
-
         // 状态与事件也不会被接受：监控数据里没有它
         await agent.SendEventAsync(AgentEventKinds.ProbeConnected, _backend.DefaultProbeId);
         await Task.Delay(200);