feat(spanner): Implement read and delete operations for notification channels (#2027)

Adds adapter methods for retrieving (Get and List) and deleting user notification
channels. Creation and update methods are omitted at this stage as channels are
currently provisioned automatically. This lays the groundwork for managing user
communication preferences.

Author: jcscottiii

lib/gcpspanner/spanneradapters/backend.go

@@ -141,6 +141,11 @@ type BackendSpannerClient interface {
 	AddUserSearchBookmark(ctx context.Context, req gcpspanner.UserSavedSearchBookmark) error
 	DeleteUserSearchBookmark(ctx context.Context, req gcpspanner.UserSavedSearchBookmark) error
 	SyncUserProfileInfo(ctx context.Context, userProfile gcpspanner.UserProfile) error
+	GetNotificationChannel(
+		ctx context.Context, channelID string, userID string) (*gcpspanner.NotificationChannel, error)
+	ListNotificationChannels(ctx context.Context, req gcpspanner.ListNotificationChannelsRequest) (
+		[]gcpspanner.NotificationChannel, *string, error)
+	DeleteNotificationChannel(ctx context.Context, channelID string, userID string) error
 }
 
 // Backend converts queries to spanner to usable entities for the backend
@@ -457,6 +462,93 @@ func (s *Backend) ListBaselineStatusCounts(
 	}, nil
 }
 
+func (s *Backend) GetNotificationChannel(ctx context.Context,
+	userID, channelID string) (*backend.NotificationChannelResponse, error) {
+	channel, err := s.client.GetNotificationChannel(ctx, channelID, userID)
+	if err != nil {
+		if errors.Is(err, gcpspanner.ErrMissingRequiredRole) {
+			return nil, errors.Join(err, backendtypes.ErrUserNotAuthorizedForAction)
+		} else if errors.Is(err, gcpspanner.ErrQueryReturnedNoResults) {
+			return nil, errors.Join(err, backendtypes.ErrEntityDoesNotExist)
+		}
+
+		return nil, err
+	}
+
+	return toBackendNotificationChannel(channel), nil
+}
+
+func (s *Backend) DeleteNotificationChannel(ctx context.Context, userID, channelID string) error {
+	err := s.client.DeleteNotificationChannel(ctx, channelID, userID)
+	if err != nil {
+		if errors.Is(err, gcpspanner.ErrMissingRequiredRole) {
+			return errors.Join(err, backendtypes.ErrUserNotAuthorizedForAction)
+		} else if errors.Is(err, gcpspanner.ErrQueryReturnedNoResults) {
+			return errors.Join(err, backendtypes.ErrEntityDoesNotExist)
+		}
+
+		return err
+	}
+
+	return nil
+}
+
+func (s *Backend) ListNotificationChannels(ctx context.Context,
+	userID string, pageSize int, pageToken *string) (*backend.NotificationChannelPage, error) {
+	listReq := gcpspanner.ListNotificationChannelsRequest{
+		UserID:    userID,
+		PageSize:  pageSize,
+		PageToken: pageToken,
+	}
+	channels, _, err := s.client.ListNotificationChannels(ctx, listReq)
+	if err != nil {
+		if errors.Is(err, gcpspanner.ErrInvalidCursorFormat) {
+			return nil, errors.Join(err, backendtypes.ErrInvalidPageToken)
+		}
+
+		return nil, err
+	}
+
+	backendChannels := make([]backend.NotificationChannelResponse, 0, len(channels))
+	for i := range channels {
+		backendChannels = append(backendChannels, *toBackendNotificationChannel(&channels[i]))
+	}
+
+	return &backend.NotificationChannelPage{
+		Data: &backendChannels,
+		Metadata: &backend.PageMetadata{
+			NextPageToken: nil,
+		},
+	}, nil
+}
+
+// toBackendNotificationChannel is a helper function to convert spanner
+// notification channel to backend notification channel.
+func toBackendNotificationChannel(channel *gcpspanner.NotificationChannel) *backend.NotificationChannelResponse {
+	if channel == nil {
+		return nil
+	}
+	// Convert spanner channel to backend channel
+	// This can be expanded to handle different channel types.
+	var value string
+	if channel.EmailConfig != nil {
+		value = channel.EmailConfig.Address
+	}
+
+	return &backend.NotificationChannelResponse{
+		Id:    channel.ID,
+		Name:  channel.Name,
+		Type:  backend.NotificationChannelResponseType(channel.Type),
+		Value: value,
+		// For now, assume all channels are enabled.
+		// We currently do not disable channels.
+		// TODO: https://github.com/GoogleChrome/webstatus.dev/issues/2021
+		Status:    backend.NotificationChannelStatusEnabled,
+		CreatedAt: channel.CreatedAt,
+		UpdatedAt: channel.UpdatedAt,
+	}
+}
+
 func (s *Backend) CreateUserSavedSearch(ctx context.Context, userID string,
 	savedSearch backend.SavedSearch) (*backend.SavedSearchResponse, error) {
 	output, err := s.client.CreateNewUserSavedSearch(ctx, gcpspanner.CreateUserSavedSearchRequest{