fix: #3628658 Correct image caption description summaries

The GLightbox and GLightbox Responsive formatters incorrectly report the Caption setting in their Caption description summaries. Use glightbox_caption_description when selecting the summary label.

Issue: https://www.drupal.org/project/glightbox/issues/3628658

Verified on Drupal 11.4.8: reproduced the original error in both formatters and checked five corrected settings combinations per formatter.

Equivalent video-module contribution: https://www.drupal.org/project/glightbox_media_video/issues/3628653

AI-Generated: Yes (Codex prepared and verified the two-line fix.)