Add J-STAGE E2E coverage and DOI fallback

[supermomonga]

Summary

• Add J-STAGE CLI E2E coverage using observed J-STAGE API/HTML behavior
• Add fallback metadata extraction from J-STAGE article citation_* meta tags when DOI lookup via the API fails
• Return a non-zero exit code when download cannot fetch metadata

Root Cause

The previous J-STAGE download path assumed DOI lookup through the J-STAGE search API could refetch metadata for search results. In production, the API can return ERR_001 for that DOI query, while the article page still exposes metadata and PDF URLs via citation meta tags. doi.org also returns an HTTP J-STAGE redirect that .NET does not automatically follow from HTTPS.

Validation

• dotnet build
• dotnet test --no-ansi --no-progress - 54 tests passed
• Verified real J-STAGE flow with search, download, and show against jstage:10.4295/audiology.20.27