Improve testing validation on data_resource_*.txt files

[Noaman-Akhtar]

Fix #2212

Describe changes proposed in this pull request:

• Add validate_resource_urls.py to check all resource URLs return HTTP 200
• Add validate_changed_resource_urls.py for incremental PR validation
• Integrate validation into CircleCI workflows (validate_all_studies.sh, validate_changed_studies.sh)
• Use GET requests (not HEAD) for data server compatibility (S3, NCBI, CloudFront)
• Implement safe git remote handling for CI environments

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