[client] Add unit tests for engine synchronization and Info flag copying

- Introduced tests for the Engine's handleSync method to verify behavior when SkipNetworkMapUpdate is true and when NetworkMap is nil.
- Added a test for the Info struct to ensure correct copying of flag values from one instance to another, while preserving unrelated fields.

client/internal/engine_sync_test.go

@@ -0,0 +1,79 @@
+package internal
+
+import (
+    "context"
+    "testing"
+
+    "golang.zx2c4.com/wireguard/wgctrl/wgtypes"
+
+    "github.com/netbirdio/netbird/client/iface"
+    "github.com/netbirdio/netbird/client/internal/peer"
+    "github.com/netbirdio/netbird/shared/management/client"
+    mgmtProto "github.com/netbirdio/netbird/shared/management/proto"
+)
+
+// Ensures handleSync exits early when SkipNetworkMapUpdate is true
+func TestEngine_HandleSync_SkipNetworkMapUpdate(t *testing.T) {
+    key, err := wgtypes.GeneratePrivateKey()
+    if err != nil {
+        t.Fatal(err)
+    }
+
+    ctx, cancel := context.WithCancel(context.Background())
+    defer cancel()
+
+    engine := NewEngine(ctx, cancel, nil, &client.MockClient{}, nil, &EngineConfig{
+        WgIfaceName:  "utun199",
+        WgAddr:       "100.70.0.1/24",
+        WgPrivateKey: key,
+        WgPort:       33100,
+        MTU:          iface.DefaultMTU,
+    }, MobileDependency{}, peer.NewRecorder("https://mgm"), nil)
+    engine.ctx = ctx
+
+    // Precondition
+    if engine.networkSerial != 0 {
+        t.Fatalf("unexpected initial serial: %d", engine.networkSerial)
+    }
+
+    resp := &mgmtProto.SyncResponse{
+        NetworkMap: &mgmtProto.NetworkMap{Serial: 42},
+        SkipNetworkMapUpdate: true,
+    }
+
+    if err := engine.handleSync(resp); err != nil {
+        t.Fatalf("handleSync returned error: %v", err)
+    }
+
+    if engine.networkSerial != 0 {
+        t.Fatalf("networkSerial changed despite SkipNetworkMapUpdate; got %d, want 0", engine.networkSerial)
+    }
+}
+
+// Ensures handleSync exits early when NetworkMap is nil
+func TestEngine_HandleSync_NilNetworkMap(t *testing.T) {
+    key, err := wgtypes.GeneratePrivateKey()
+    if err != nil {
+        t.Fatal(err)
+    }
+
+    ctx, cancel := context.WithCancel(context.Background())
+    defer cancel()
+
+    engine := NewEngine(ctx, cancel, nil, &client.MockClient{}, nil, &EngineConfig{
+        WgIfaceName:  "utun198",
+        WgAddr:       "100.70.0.2/24",
+        WgPrivateKey: key,
+        WgPort:       33101,
+        MTU:          iface.DefaultMTU,
+    }, MobileDependency{}, peer.NewRecorder("https://mgm"), nil)
+    engine.ctx = ctx
+
+    resp := &mgmtProto.SyncResponse{NetworkMap: nil}
+
+    if err := engine.handleSync(resp); err != nil {
+        t.Fatalf("handleSync returned error: %v", err)
+    }
+}
+
+