fix(openai): 统一专属倍率计费链路并补齐回归测试

抽取共享的用户分组专属倍率解析器，统一缓存、singleflight 与回退逻辑。\n\n让 OpenAI 独立计费链路复用专属倍率解析，修复 usage 记录与实际扣费未命中用户专属倍率的问题。\n\n补齐 OpenAI 计费与解析器单元测试，并修复全量回归中暴露的 lint 阻塞项。\n\nCo-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/internal/service/user_group_rate_resolver_test.go

@@ -0,0 +1,83 @@
+package service
+
+import (
+	"context"
+	"testing"
+	"time"
+
+	gocache "github.com/patrickmn/go-cache"
+	"github.com/stretchr/testify/require"
+)
+
+type userGroupRateResolverRepoStub struct {
+	UserGroupRateRepository
+
+	rate  *float64
+	err   error
+	calls int
+}
+
+func (s *userGroupRateResolverRepoStub) GetByUserAndGroup(ctx context.Context, userID, groupID int64) (*float64, error) {
+	s.calls++
+	if s.err != nil {
+		return nil, s.err
+	}
+	return s.rate, nil
+}
+
+func TestNewUserGroupRateResolver_Defaults(t *testing.T) {
+	resolver := newUserGroupRateResolver(nil, nil, 0, nil, "")
+
+	require.NotNil(t, resolver)
+	require.NotNil(t, resolver.cache)
+	require.Equal(t, defaultUserGroupRateCacheTTL, resolver.cacheTTL)
+	require.NotNil(t, resolver.sf)
+	require.Equal(t, "service.gateway", resolver.logComponent)
+}
+
+func TestUserGroupRateResolverResolve_FallbackForNilResolverAndInvalidIDs(t *testing.T) {
+	var nilResolver *userGroupRateResolver
+	require.Equal(t, 1.4, nilResolver.Resolve(context.Background(), 101, 202, 1.4))
+
+	resolver := newUserGroupRateResolver(nil, nil, time.Second, nil, "service.test")
+	require.Equal(t, 1.4, resolver.Resolve(context.Background(), 0, 202, 1.4))
+	require.Equal(t, 1.4, resolver.Resolve(context.Background(), 101, 0, 1.4))
+}
+
+func TestUserGroupRateResolverResolve_InvalidCacheEntryLoadsRepoAndCaches(t *testing.T) {
+	resetGatewayHotpathStatsForTest()
+
+	rate := 1.7
+	repo := &userGroupRateResolverRepoStub{rate: &rate}
+	cache := gocache.New(time.Minute, time.Minute)
+	cache.Set("101:202", "bad-cache", time.Minute)
+	resolver := newUserGroupRateResolver(repo, cache, time.Minute, nil, "service.test")
+
+	got := resolver.Resolve(context.Background(), 101, 202, 1.2)
+	require.Equal(t, rate, got)
+	require.Equal(t, 1, repo.calls)
+
+	cached, ok := cache.Get("101:202")
+	require.True(t, ok)
+	require.Equal(t, rate, cached)
+
+	hit, miss, load, _, fallback := GatewayUserGroupRateCacheStats()
+	require.Equal(t, int64(0), hit)
+	require.Equal(t, int64(1), miss)
+	require.Equal(t, int64(1), load)
+	require.Equal(t, int64(0), fallback)
+}
+
+func TestGatewayServiceGetUserGroupRateMultiplier_FallbacksAndUsesExistingResolver(t *testing.T) {
+	var nilSvc *GatewayService
+	require.Equal(t, 1.3, nilSvc.getUserGroupRateMultiplier(context.Background(), 101, 202, 1.3))
+
+	rate := 1.9
+	repo := &userGroupRateResolverRepoStub{rate: &rate}
+	resolver := newUserGroupRateResolver(repo, nil, time.Minute, nil, "service.gateway")
+	svc := &GatewayService{userGroupRateResolver: resolver}
+
+	got := svc.getUserGroupRateMultiplier(context.Background(), 101, 202, 1.2)
+	require.Equal(t, rate, got)
+	require.Equal(t, 1, repo.calls)
+}