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/handler/admin/account_today_stats_cache.go

@@ -1,7 +1,6 @@
 package admin
 
 import (
-	"sort"
 	"strconv"
 	"strings"
 	"time"
@@ -9,26 +8,6 @@ import (
 
 var accountTodayStatsBatchCache = newSnapshotCache(30 * time.Second)
 
-func normalizeAccountIDList(accountIDs []int64) []int64 {
-	if len(accountIDs) == 0 {
-		return nil
-	}
-	seen := make(map[int64]struct{}, len(accountIDs))
-	out := make([]int64, 0, len(accountIDs))
-	for _, id := range accountIDs {
-		if id <= 0 {
-			continue
-		}
-		if _, ok := seen[id]; ok {
-			continue
-		}
-		seen[id] = struct{}{}
-		out = append(out, id)
-	}
-	sort.Slice(out, func(i, j int) bool { return out[i] < out[j] })
-	return out
-}
-
 func buildAccountTodayStatsBatchCacheKey(accountIDs []int64) string {
 	if len(accountIDs) == 0 {
 		return "accounts_today_stats_empty"