Fix flaky lease expiry test

[asg0451]

Add a guard to the lease sweeper to prevent expiring extended leases due to stale expiry index entries.

The expire_due_leases function could incorrectly expire a lease that had been extended. This happened if a stale expiry index entry (pointing to an old, past expiry time) was processed before the lease's actual entry (which had been updated with a new, future expiry). The guard checks the lease's true expiry time and, if it's in the future, only deletes the stale index key, resolving the race condition and fixing the flaky extend_existing_lease_updates_expiry_index test.

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[cursor]

Cursor Agent can help with this pull request. Just @cursor in comments and I'll start working on changes in this branch.
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[github-actions]

Criterion benchmarks delta (ms; lower is better)

Benchmark	PR (ms)	Master (ms)	Δ (ms)	Δ (%)
e2e_rpc/rpc_add_batch_200	0.621	0.609	0.012	+2.0%
e2e_rpc/rpc_poll_batch_200	50.741	51.928	-1.187	-2.3%
e2e_rpc/rpc_remove_each_200	24.688	24.758	-0.070	-0.3%
e2e_rpc/rpc_stress_like_p2_a2_items_200	1170.986	1202.766	-31.780	-2.6%
storage_add/add_available_500	8.153	8.295	-0.142	-1.7%
storage_poll/get_next_available_entries_500	10.115	10.528	-0.413	-3.9%
storage_remove/remove_in_progress_500	20.242	20.622	-0.380	-1.8%