test(engine): align heartbeat task-create assertion with FN-5060 dedup shape

51fc82619 (FN-5060 dedup) added sourceParentTaskId / sourceMetadata /
sourceRunId to the heartbeat-spawned task source but didn't update this
test, so the assertion was failing on `sourceRunId: undefined` matching
against the real `"run-001"`. Drop the over-tight key — we still verify
sourceType and sourceAgentId; sourceRunId is covered separately.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

packages/engine/src/__tests__/heartbeat-executor.test.ts

@@ -3181,7 +3181,6 @@ describe("executeHeartbeat", () => {
         source: expect.objectContaining({
           sourceType: "agent_heartbeat",
           sourceAgentId: "agent-001",
-          sourceRunId: undefined,
         }),
       }), expect.objectContaining({ settings: { autoSummarizeTitles: false } }));
     });