fix: credit changelog co-authors

[andygrove]

Summary

• credit every recognized commit author on multi-author pull requests
• include squashed co-authors in the contributor count and credits table
• tolerate commits whose author is not linked to a GitHub account
• retain Ballista-specific dependency filtering and safe subprocess usage

This ports the recent attribution improvement from apache/datafusion-comet#6358 while preserving the improvements already present in Ballista.

Testing

• uvx --from ruff==0.14.13 ruff check dev/release/generate-changelog.py
• uvx --from ruff==0.14.13 ruff format --check dev/release/generate-changelog.py
• mocked co-author and nullable-author attribution tests
• generated the 54.0.0..54.1.0 changelog against the live GitHub API