feat(get-changed-files): support push events via before/after SHAs

[danadajian]

Summary

• The get-changed-files helper previously only worked on pull_request and merge_group events. It now also supports push events by diffing the before and after SHAs from the push payload.
• Changed files are gathered by paginating through all commits in the push range and fetching each commit's files in parallel, then deduplicating by filename (last status wins). This avoids the 300-file cap on the compare endpoint's file list.
• The file processing logic (handling renamed files and the ignore_deleted flag) was extracted into a shared helper to eliminate duplication between the PR and push paths.
• Unit tests cover the push path: single commit, multiple commits, commit pagination, ignore_deleted, pattern filtering, and cross-commit deduplication.

[eg-oss-ci]

🎉 This PR is included in version 1.90.0 🎉

The release is available on GitHub release

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