fix: resolve golangci-lint issues

- Fix errcheck: defer rows.Close() with nolint
- Fix errcheck: type assertion with ok check in channel cache
- Fix staticcheck ST1005: lowercase error string
- Fix staticcheck SA5011: nil check cost before use in openai gateway
- Fix gofmt: format chatcompletions_to_responses.go

backend/internal/repository/channel_repo.go

@@ -443,7 +443,7 @@ func (r *channelRepository) GetGroupPlatforms(ctx context.Context, groupIDs []in
 	if err != nil {
 		return nil, fmt.Errorf("get group platforms: %w", err)
 	}
-	defer rows.Close()
+	defer rows.Close() //nolint:errcheck
 
 	result := make(map[int64]string, len(groupIDs))
 	for rows.Next() {