fix: add PR number context to file-content fetch failure warning

[MkDev11]

Summary

• Adds optional pr_number context to the existing warning in _fetch_file_contents_with_base_batch when file-content GraphQL fetch returns None.
• Threads pr.number from scoring into fetch_file_contents_with_base(...) so logs identify the exact failed PR.
• Keeps behavior intentionally unchanged:
  • no tuple-return API changes
  • no skip-vs-zero branching changes
  • no None vs [] return-contract changes
  • no new test file
• Net diff is minimal (13 lines total: +10 / -3 across 2 files).

Related Issues

Closes #710
Supersedes #645

Type of Change

• Bug fix
• New feature
• Refactor
• Documentation
• Other (describe below)

Testing

• Tests added/updated

• Manually tested

• Ran:

  • uv run pytest tests/utils/test_github_api_tools.py tests/validator/test_base_score.py

• Result:

  • 82 passed

Checklist

• Code follows project style guidelines
• Self-review completed
• Changes are documented (if applicable)