Improve GWAS Catalog scraping robustness and documentation

[Copilot]

The GWAS Catalog downloader had brittle scraping helpers and minimal documentation, leading to missed files and unclear behavior.

• Scraping helpers
  • Added explicit filtering for invalid links and directory detection; retain subdirectories while applying file patterns.
  • Prefer first harmonised directory when present; gracefully fall back to base files.
• Documentation
  • Documented harmonised-preferred behavior and noted pattern=NULL handling in scraper helpers.
• Tests
  • Added mocked tests covering .scrape filtering and scrape_dir harmonised path handling.

Example:

# Returns directories plus files matching pattern
tidyGWAS:::`.scrape`("http://example.com", pattern = "\\.(tsv|yaml)$")
# -> c("harmonised/", "results.tsv")

Original prompt

Identify potential bugs and instances of lacking documentation

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