refactor(hub): decode each agent response into a new struct

The WebSocket path reused sys.data, and CBOR leaves omitted fields
untouched, so stale values carried over (e.g. Details re-saved every
poll, SystemdServicesUpdated stuck true, old map entries kept).

sys.data is now set only by the regular update for alerts. Removes the
Wi-Fi-only workaround in UnmarshalResponse.

internal/hub/systems/ws_fresh_data_test.go

@@ -0,0 +1,92 @@
+//go:build testing
+
+package systems
+
+import (
+	"net/http"
+	"net/http/httptest"
+	"strings"
+	"testing"
+	"time"
+
+	"github.com/blang/semver"
+	"github.com/fxamacker/cbor/v2"
+	"github.com/henrygd/beszel/internal/common"
+	esystem "github.com/henrygd/beszel/internal/entities/system"
+	"github.com/henrygd/beszel/internal/hub/ws"
+	"github.com/lxzan/gws"
+	"github.com/stretchr/testify/require"
+)
+
+// sequenceDataClient answers each GetData request with the next queued payload.
+type sequenceDataClient struct {
+	gws.BuiltinEventHandler
+	responses chan esystem.CombinedData
+}
+
+func (c *sequenceDataClient) OnMessage(conn *gws.Conn, message *gws.Message) {
+	defer message.Close()
+	var req common.HubRequest[cbor.RawMessage]
+	if err := cbor.Unmarshal(message.Bytes(), &req); err != nil || req.Action != common.GetData {
+		return
+	}
+	data, _ := cbor.Marshal(<-c.responses)
+	response, _ := cbor.Marshal(common.AgentResponse{Id: req.Id, Data: data})
+	_ = conn.WriteMessage(gws.OpcodeBinary, response)
+}
+
+// Fields the agent omits must not carry over from a previous response.
+func TestFetchDataViaWebSocketDoesNotRetainOmittedFields(t *testing.T) {
+	connections := make(chan *ws.WsConn, 1)
+	upgrader := gws.NewUpgrader(&monitorSyncServer{}, nil)
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		conn, err := upgrader.Upgrade(w, r)
+		if err != nil {
+			t.Error(err)
+			return
+		}
+		wsConn := ws.NewWsConnection(conn, semver.MustParse("0.20.0"))
+		conn.Session().Store("wsConn", wsConn)
+		connections <- wsConn
+		conn.ReadLoop()
+	}))
+	t.Cleanup(server.Close)
+
+	client := &sequenceDataClient{responses: make(chan esystem.CombinedData, 2)}
+	client.responses <- esystem.CombinedData{
+		Details:                &esystem.Details{Hostname: "host"},
+		SystemdServicesUpdated: true,
+		Stats:                  esystem.Stats{Batteries: map[string]uint8{"BAT0": 50, "BAT1": 60}},
+		Info:                   esystem.Info{WiFi: map[string]esystem.WiFi{"wlan0": {SSID: "home"}}},
+	}
+	client.responses <- esystem.CombinedData{
+		Stats: esystem.Stats{Batteries: map[string]uint8{"BAT0": 40}},
+	}
+	conn, _, err := gws.NewClient(client, &gws.ClientOption{Addr: "ws" + strings.TrimPrefix(server.URL, "http")})
+	require.NoError(t, err)
+	t.Cleanup(func() { _ = conn.NetConn().Close() })
+	go conn.ReadLoop()
+
+	var sys *System
+	select {
+	case wsConn := <-connections:
+		sys = &System{WsConn: wsConn}
+	case <-time.After(3 * time.Second):
+		t.Fatal("websocket connection was not established")
+	}
+
+	first, err := sys.fetchDataFromAgent(common.DataRequestOptions{})
+	require.NoError(t, err)
+	require.NotNil(t, first.Details)
+	require.Len(t, first.Stats.Batteries, 2)
+
+	second, err := sys.fetchDataFromAgent(common.DataRequestOptions{})
+	require.NoError(t, err)
+	require.Nil(t, second.Details)
+	require.False(t, second.SystemdServicesUpdated)
+	require.Equal(t, map[string]uint8{"BAT0": 40}, second.Stats.Batteries)
+	require.Empty(t, second.Info.WiFi)
+
+	// the first result is not mutated by the second fetch
+	require.Len(t, first.Stats.Batteries, 2)
+}