feat(runtime): track node lifecycle and QUIC link state

backend/fleet/src/Fleet.Server.Link/NodeConnection.Receive.cs

@@ -64,6 +64,17 @@ internal sealed partial class NodeConnection
             case FleetMethods.Heartbeat:
                 await HandleHeartbeatAsync(message, ct);
                 return;
+            case FleetMethods.Stopping:
+                try
+                {
+                    string? reason = message.Params?.Deserialize(FleetRpcContext.Default.StoppingParams)?.Reason;
+                    _closeReason = reason is "updating" ? "updating" : "stopping";
+                }
+                catch (JsonException)
+                {
+                    _closeReason = "stopping";
+                }
+                return;
         }
 
         if (_node!.Status != NodeStatus.Active)