fix: Messages() routing refactor and subscription group test coverage

- Refactor OpenAI Messages() routing: pre-compute dispatch model using
  resolveOpenAIMessagesDispatchMappedModel + NormalizeOpenAICompatRequestedModel
  instead of try-fail-retry pattern with gin context passing
- Remove openai_messages_fallback_model context anti-pattern
- Use effectiveMappedModel directly for forward default mapped model
- Add 3 subscription group tests covering all branch paths:
  _Blocked (no active subscription → SUBSCRIPTION_REQUIRED),
  _RequiresRepo (nil repo → SUBSCRIPTION_REPOSITORY_UNAVAILABLE),
  _AllowsActiveSubscription (valid subscription → success)

backend/internal/service/admin_service_apikey_test.go

@@ -216,6 +216,29 @@ func (s *groupRepoStubForGroupUpdate) UpdateSortOrders(context.Context, []GroupS
 	panic("unexpected")
 }
 
+type userSubRepoStubForGroupUpdate struct {
+	userSubRepoNoop
+	getActiveSub  *UserSubscription
+	getActiveErr  error
+	called        bool
+	calledUserID  int64
+	calledGroupID int64
+}
+
+func (s *userSubRepoStubForGroupUpdate) GetActiveByUserIDAndGroupID(_ context.Context, userID, groupID int64) (*UserSubscription, error) {
+	s.called = true
+	s.calledUserID = userID
+	s.calledGroupID = groupID
+	if s.getActiveErr != nil {
+		return nil, s.getActiveErr
+	}
+	if s.getActiveSub == nil {
+		return nil, ErrSubscriptionNotFound
+	}
+	clone := *s.getActiveSub
+	return &clone, nil
+}
+
 // ---------------------------------------------------------------------------
 // Tests
 // ---------------------------------------------------------------------------
@@ -408,17 +431,52 @@ func TestAdminService_AdminUpdateAPIKeyGroupID_NonExclusiveGroup_NoAllowedGroupU
 func TestAdminService_AdminUpdateAPIKeyGroupID_SubscriptionGroup_Blocked(t *testing.T) {
 	existing := &APIKey{ID: 1, UserID: 42, Key: "sk-test", GroupID: nil}
 	apiKeyRepo := &apiKeyRepoStubForGroupUpdate{key: existing}
-	groupRepo := &groupRepoStubForGroupUpdate{group: &Group{ID: 10, Name: "Sub", Status: StatusActive, IsExclusive: true, SubscriptionType: SubscriptionTypeSubscription}}
+	groupRepo := &groupRepoStubForGroupUpdate{group: &Group{ID: 10, Name: "Sub", Status: StatusActive, IsExclusive: false, SubscriptionType: SubscriptionTypeSubscription}}
+	userRepo := &userRepoStubForGroupUpdate{}
+	userSubRepo := &userSubRepoStubForGroupUpdate{getActiveErr: ErrSubscriptionNotFound}
+	svc := &adminServiceImpl{apiKeyRepo: apiKeyRepo, groupRepo: groupRepo, userRepo: userRepo, userSubRepo: userSubRepo}
+
+	// 无有效订阅时应拒绝绑定
+	_, err := svc.AdminUpdateAPIKeyGroupID(context.Background(), 1, int64Ptr(10))
+	require.Error(t, err)
+	require.Equal(t, "SUBSCRIPTION_REQUIRED", infraerrors.Reason(err))
+	require.True(t, userSubRepo.called)
+	require.Equal(t, int64(42), userSubRepo.calledUserID)
+	require.Equal(t, int64(10), userSubRepo.calledGroupID)
+	require.False(t, userRepo.addGroupCalled)
+}
+
+func TestAdminService_AdminUpdateAPIKeyGroupID_SubscriptionGroup_RequiresRepo(t *testing.T) {
+	existing := &APIKey{ID: 1, UserID: 42, Key: "sk-test", GroupID: nil}
+	apiKeyRepo := &apiKeyRepoStubForGroupUpdate{key: existing}
+	groupRepo := &groupRepoStubForGroupUpdate{group: &Group{ID: 10, Name: "Sub", Status: StatusActive, IsExclusive: false, SubscriptionType: SubscriptionTypeSubscription}}
 	userRepo := &userRepoStubForGroupUpdate{}
 	svc := &adminServiceImpl{apiKeyRepo: apiKeyRepo, groupRepo: groupRepo, userRepo: userRepo}
 
-	// userSubRepo is nil → SUBSCRIPTION_REPOSITORY_UNAVAILABLE
 	_, err := svc.AdminUpdateAPIKeyGroupID(context.Background(), 1, int64Ptr(10))
 	require.Error(t, err)
 	require.Equal(t, "SUBSCRIPTION_REPOSITORY_UNAVAILABLE", infraerrors.Reason(err))
 	require.False(t, userRepo.addGroupCalled)
 }
 
+func TestAdminService_AdminUpdateAPIKeyGroupID_SubscriptionGroup_AllowsActiveSubscription(t *testing.T) {
+	existing := &APIKey{ID: 1, UserID: 42, Key: "sk-test", GroupID: nil}
+	apiKeyRepo := &apiKeyRepoStubForGroupUpdate{key: existing}
+	groupRepo := &groupRepoStubForGroupUpdate{group: &Group{ID: 10, Name: "Sub", Status: StatusActive, IsExclusive: true, SubscriptionType: SubscriptionTypeSubscription}}
+	userRepo := &userRepoStubForGroupUpdate{}
+	userSubRepo := &userSubRepoStubForGroupUpdate{
+		getActiveSub: &UserSubscription{ID: 99, UserID: 42, GroupID: 10},
+	}
+	svc := &adminServiceImpl{apiKeyRepo: apiKeyRepo, groupRepo: groupRepo, userRepo: userRepo, userSubRepo: userSubRepo}
+
+	got, err := svc.AdminUpdateAPIKeyGroupID(context.Background(), 1, int64Ptr(10))
+	require.NoError(t, err)
+	require.True(t, userSubRepo.called)
+	require.NotNil(t, got.APIKey.GroupID)
+	require.Equal(t, int64(10), *got.APIKey.GroupID)
+	require.False(t, userRepo.addGroupCalled)
+}
+
 func TestAdminService_AdminUpdateAPIKeyGroupID_ExclusiveGroup_AllowedGroupAddFails_ReturnsError(t *testing.T) {
 	existing := &APIKey{ID: 1, UserID: 42, Key: "sk-test", GroupID: nil}
 	apiKeyRepo := &apiKeyRepoStubForGroupUpdate{key: existing}