Unicron 4835 handle edge cases prod

[lukaszgryglicki]

• Handle commend body too big (> 64K characters - can happen if PR has a lot of commits) in both python and golang backends.
• Handle PRs from forks with > 250 commits with graceful fallack to REST (both python and golang backends).
• Correctly locate PR head SHA using quick API call and fallback to getting last PR's commit SHA (also both backends).
• Log number of distinct commit SHAs, authors (ids, logins, emails, names) in both backends so we can see how many commit summaries were there and how many actual commits (one commit can have > 1 authors) - both backends.
• Handle private repos and forks correctly in both backends with fallbacks on errors.

cc @mlehotskylf @ahmedomosanya @jarias-lfx - this is for #4835.

Signed-off-by: Lukasz Gryglicki lgryglicki@cncf.io

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✨ Finishing touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch unicron-4835-handle-edge-cases-prod

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Pull Request Overview

This PR enhances the CLA backend to gracefully handle edge cases in PR processing across both Python and Go implementations. The changes ensure robust handling of large comment bodies, PRs with many commits from forks, and edge cases in commit SHA retrieval.

Key Changes:

• Implemented comment body size limiting to prevent exceeding the 64KB GitHub limit by trimming SHA lists when comments become too large
• Refactored PR commit fetching to use merge-base history traversal, supporting PRs with > 250 commits and graceful REST API fallback
• Added comprehensive logging of distinct commit metadata (SHAs, author IDs, logins, emails, names) to track commit summaries vs actual commits

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

File	Description
cla-backend/cla/utils.py	Added comment trimming logic to handle oversized comment bodies and SHA list truncation
cla-backend/cla/models/github_models.py	Rewrote GraphQL commit iteration to use merge-base history traversal with fallback handling and enhanced logging
cla-backend-go/github/github_repository.go	Mirrored Python changes with merge-base history traversal, comment trimming, improved error handling, and consistent logging

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