FN-6394: relax non-progress userPaused assertion

Update the non-progress churn reliability test to match the Move-Task contract for never-user-paused tasks.

- Treat undefined userPaused as not user-paused instead of requiring false.
- Document why engine rebounds should not write userPaused for never-user-paused tasks.

Files changed:
 .../src/__tests__/reliability-interactions/non-progress-churn.test.ts | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

Fusion-Task-Id: FN-6394

Fusion-Task-Lineage: 59554c89-4e32-490f-88eb-d22ee7fdca31

packages/engine/src/__tests__/reliability-interactions/non-progress-churn.test.ts

@@ -214,7 +214,9 @@ describe("reliability interactions: non-progress churn", () => {
     expect(task.status).toBe("queued");
     expect(task.column).toBe("todo");
     expect(task.paused).toBe(false);
-    expect(task.userPaused).toBe(false);
+    // FN-6252 / Move-Task contract: engine rebounds do not write userPaused,
+    // so a never-user-paused task remains undefined while still not user-paused.
+    expect(task.userPaused).not.toBe(true);
     expect(task.pausedReason).toBeNull();
     expect(task.stuckKillCount).toBe(7);
     expect(task.steps).toEqual([{ name: "Implement", status: "in-progress" }]);