fix: #3628653 Correct the caption description summary

The Caption description summary incorrectly reports the Caption setting. Use glightbox_caption_description when selecting its summary label.

Issue: https://www.drupal.org/project/glightbox_media_video/issues/3628653

Verified on Drupal 11.4.8: reproduced the original error and checked five corrected settings combinations, including different caption/description choices and an empty description.

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