refactor: 消除重复的 normalizeAccountIDList，补充 PR#754 新增组件的单元测试

- 删除 account_today_stats_cache.go 中重复的 normalizeAccountIDList，统一使用 id_list_utils.go 的 normalizeInt64IDList
- 新增 snapshot_cache_test.go：覆盖 snapshotCache、buildETagFromAny、parseBoolQueryWithDefault
- 新增 id_list_utils_test.go：覆盖 normalizeInt64IDList、buildAccountTodayStatsBatchCacheKey
- 新增 ops_query_mode_test.go：覆盖 shouldFallbackOpsPreagg、cloneOpsFilterWithMode

backend/internal/service/ops_query_mode_test.go

@@ -0,0 +1,66 @@
+//go:build unit
+
+package service
+
+import (
+	"errors"
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestShouldFallbackOpsPreagg(t *testing.T) {
+	preaggErr := ErrOpsPreaggregatedNotPopulated
+	otherErr := errors.New("some other error")
+
+	autoFilter := &OpsDashboardFilter{QueryMode: OpsQueryModeAuto}
+	rawFilter := &OpsDashboardFilter{QueryMode: OpsQueryModeRaw}
+	preaggFilter := &OpsDashboardFilter{QueryMode: OpsQueryModePreagg}
+
+	tests := []struct {
+		name   string
+		filter *OpsDashboardFilter
+		err    error
+		want   bool
+	}{
+		{"auto mode + preagg error => fallback", autoFilter, preaggErr, true},
+		{"auto mode + other error => no fallback", autoFilter, otherErr, false},
+		{"auto mode + nil error => no fallback", autoFilter, nil, false},
+		{"raw mode + preagg error => no fallback", rawFilter, preaggErr, false},
+		{"preagg mode + preagg error => no fallback", preaggFilter, preaggErr, false},
+		{"nil filter => no fallback", nil, preaggErr, false},
+		{"wrapped preagg error => fallback", autoFilter, errors.Join(preaggErr, otherErr), true},
+	}
+
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			got := shouldFallbackOpsPreagg(tc.filter, tc.err)
+			require.Equal(t, tc.want, got)
+		})
+	}
+}
+
+func TestCloneOpsFilterWithMode(t *testing.T) {
+	t.Run("nil filter returns nil", func(t *testing.T) {
+		require.Nil(t, cloneOpsFilterWithMode(nil, OpsQueryModeRaw))
+	})
+
+	t.Run("cloned filter has new mode", func(t *testing.T) {
+		groupID := int64(42)
+		original := &OpsDashboardFilter{
+			StartTime: time.Now(),
+			EndTime:   time.Now().Add(time.Hour),
+			Platform:  "anthropic",
+			GroupID:   &groupID,
+			QueryMode: OpsQueryModeAuto,
+		}
+
+		cloned := cloneOpsFilterWithMode(original, OpsQueryModeRaw)
+		require.Equal(t, OpsQueryModeRaw, cloned.QueryMode)
+		require.Equal(t, OpsQueryModeAuto, original.QueryMode, "original should not be modified")
+		require.Equal(t, original.Platform, cloned.Platform)
+		require.Equal(t, original.StartTime, cloned.StartTime)
+		require.Equal(t, original.GroupID, cloned.GroupID)
+	})
+}