all checkpoints should throw LeaseExpired when applicable

[virajmehta]

We were not failing tasks when their lease had expired at the time they are writing checkpoints. We now throw a control error when a task attempts to write a checkpoint with an expired lease.

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Note

Medium Risk
Changes core Postgres lease/checkpoint functions and worker control-flow handling; mistakes could cause unexpected task termination or lock contention under concurrency.

Overview
Checkpoints/heartbeats now enforce active leases. durable.set_task_checkpoint_state and durable.extend_claim were updated to lock in task -> run order and to validate that the run is still running with an unexpired claim_expires_at; if not, they raise SQLSTATE AB002 (added via a new migration).

Runtime behavior changes. Rust adds ControlFlow::LeaseExpired (mapped from AB002) and the worker treats it as non-fatal control flow (logs and stops without double-failing the run). Tests add SleepThenCheckpointTask plus new lease-expiry cases asserting no checkpoints are written after lease expiry; benches add hierarchical parent/child/grandchild tasks for stress testing.

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