Fix handling GitHub PR commits on rebased PRs (prod)

[lukaszgryglicki]

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

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Pull request overview

Fixes PR commit enumeration for rebased / large GitHub pull requests by switching commit listing logic to a compare-based approach (base SHA … head SHA) and tightening REST fallback behavior when commit lists may be truncated.

Changes:

• Replace GraphQL-based PR commit traversal with compare-endpoint pagination in Python and Go implementations.
• Block “unsafe” REST PR-commits fallback when commit counts are unknown or potentially truncated (250 limit).
• Add/adjust unit tests and add helper CLI scripts to inspect PR commit enumeration results.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
utils/get_pr_commits.py	Adds a Python helper script to compare GraphQL/REST/compare-based commit enumeration outputs.
tests/functional/cypress/e2e/v4/signatures.cy.ts	Minor assertion style change (gte(0)), no functional impact.
cla-backend/cla/tests/unit/test_github_models.py	Adds unit tests covering compare retry behavior and blocking unsafe REST fallback.
cla-backend/cla/models/github_models.py	Introduces iter_pr_commits_compare() and routes PR commit author extraction through compare-based enumeration; adjusts check-run summary text.
cla-backend-go/github/github_repository.go	Implements compare-based commit listing with retries + parallel page fetching; adds compare-based author extraction and guarded REST fallback.
cla-backend-go/github/github_repository_test.go	Adds tests for compare retry behavior and page-order preservation under parallel fetching.
cla-backend-go/cmd/get_pr_commits/main.go	Adds a Go helper CLI for listing PR commits using compare-based enumeration.