zapslog: add test coverage - WithGroup in WithGroup

Signed-off-by: junya koyama <arukiidou@yahoo.co.jp>

exp/zapslog/handler.go

@@ -197,7 +197,7 @@ func (h *Handler) WithAttrs(attrs []slog.Attr) slog.Handler {
 func (h *Handler) WithGroup(group string) slog.Handler {
 	cloned := *h
 	if h.holdGroup != "" {
-		cloned.core = h.core.With([]zapcore.Field{zap.Namespace(group)})
+		cloned.core = h.core.With([]zapcore.Field{zap.Namespace(h.holdGroup)})
 	}
 	cloned.holdGroup = group
 	return &cloned

exp/zapslog/handler_test.go

@@ -190,6 +190,23 @@ func TestInlineGroup(t *testing.T) {
 func TestWithGroup(t *testing.T) {
 	t.Parallel()
 	fac, observedLogs := observer.New(zapcore.DebugLevel)
+	t.Run("group-in-group", func(t *testing.T) {
+		sl := slog.New(NewHandler(fac))
+		sl.With("a", "b").WithGroup("G").WithGroup("in").Info("msg", "c", "d")
+
+		logs := observedLogs.TakeAll()
+		require.Len(t, logs, 1, "Expected exactly one entry to be logged")
+		entry := logs[0]
+		assert.Equal(t, "", entry.LoggerName, "Unexpected logger name")
+		assert.Equal(t, map[string]any{
+			"G": map[string]any{
+				"in": map[string]any{
+					"c": "d",
+				},
+			},
+			"a": "b",
+		}, logs[0].ContextMap(), "Unexpected context")
+	})
 	t.Run("empty-group-record", func(t *testing.T) {
 		sl := slog.New(NewHandler(fac))
 		sl.With("a", "b").WithGroup("G").With("c", "d").WithGroup("H").Info("msg")