fix: preserve changelog entries with missing ATT&CK URLs

[jondricek]

Summary

• Warn with the STIX ID, ATT&CK ID, and name when the first external reference has no usable ATT&CK website URL.
• Keep affected changelog entries as plain text, including revocation and data-component entries, instead of emitting broken links.
• Check only the first external reference for an object URL and add regression coverage for missing and invalid URLs.

Validation

• Full pytest suite: 297 passed, 4 skipped.
• ruff check . and ruff format --check . passed.
• Pre-commit hooks passed.

The repository has no GitHub Actions workflow triggered by pushes to this feature branch; PR workflows will run on this pull request.

[sonarqubecloud]

Quality Gate passed

Issues
1 New issue
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud