Adds test coverage for merge queue and heartbeat handoff interactions (FN-5444), including source metadata expectations in heartbeat executor tests, merge handoff coverage gaps, and reuse scenarios in the merger worktree integration tests. Fusion-Task-Id: FN-5444 Fusion-Task-Lineage: 45e1b43f-8ae3-46ba-a5ee-25e8c661e753 Co-authored-by: Fusion (runfusion.ai) <noreply@runfusion.ai> Fusion-Task-Id: FN-5444