fix: skip invalid deviantart download targets without src

[harshang03]

Summary

• avoid hard failures when DeviantArt download payloads do not include a media source URL
• treat non-downloadable payloads as authorization failures and continue extraction safely
• add shared media-source resolution helper and apply it to other extractors that previously assumed src was always present
• add focused regression tests for DeviantArt source-missing behavior

What changed

• gallery_dl/extractor/deviantart.py: skip invalid targets, validate download payloads, and handle restricted downloads without crashing
• gallery_dl/extractor/common.py: add media_source() helper for defensive URL extraction
• gallery_dl/extractor/shopify.py: skip image entries without source URL
• gallery_dl/extractor/pexels.py: guard missing video/image source links
• gallery_dl/extractor/newgrounds.py: guard missing source URLs in video format selection
• test/test_deviantart.py: regression tests for missing-source and authorization paths

Test plan

• ./.venv/bin/python -m pytest test/test_deviantart.py test/test_job.py

Fixes #9072