Refactor paginated_check_runs to use lazy pagination

[katestud]

Context

The RefreshCheckRunsJob refreshes all checks associated with a particular commit via refresh_check_runs!. That method relies on paginated_check_runs, which currently loads all paginated results into memory before processing them. This can result in memory bloat for commits with a large number of check runs (e.g., hundreds of check runs across multiple pages).

Changes

This PR updates paginated_check_runs to process each page lazily by yielding batches to the caller as they're fetched, rather than accumulating all results in memory first.

Testing

This also adds test coverage for the pagination case, as existing test coverage only handled single-page responses.