feat(engine): add commit iteration support for pull request evaluation

[sachin9058]

Summary

This PR introduces basic support for iterating over all commits in a pull request during evaluation.

Currently, Minder evaluates only the final (HEAD) state of a pull request. This change lays the groundwork for future commit-level policies by exposing commit data such as SHA and message across all commits in the PR.

The implementation:

• Adds commit iteration logic in the evaluation flow using a provider-agnostic interface (commitLister)
• Supports pagination when fetching commits from the provider
• Logs commit metadata (SHA and message) in a structured way for debugging and future extension

This change is intentionally scoped to be foundational and does not introduce any policy enforcement yet.

Fixes #2176

Testing

• Verified that commit iteration works across multiple pages of commits
• Confirmed that the logic is a no-op for providers that do not implement commitLister
• Ran full test suite:
  go test ./...
• Ensured no existing functionality is affected

[sachin9058]

Note: Observed a data race in TestNatsChannel related to NATS JetStream internals. This appears to be upstream and unrelated to the changes in this PR.

[sachin9058]

@evankanderson This PR is intentionally scoped as a foundational step toward commit-level evaluation.

It introduces commit iteration support across pull requests while keeping the existing behavior unchanged. This allows future enhancements such as commit-level policy checks (e.g., content, message format, metadata) to build on top of this without expanding the scope of this PR.

[coveralls]

Coverage is 60.096% — sachin9058:feat/commit-iteration-support into mindersec:main. No base build found for mindersec:main.

[github-actions]

This PR needs additional information before we can continue. It is now marked as stale because it has been open for 30 days with no activity. Please provide the necessary details to continue or it will be closed in 30 days.