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.