added more flexibility to public interface around heartbeating

[virajmehta]

Note

Medium Risk
Public API changes to TaskContext::step and heartbeat plumbing could require downstream updates and may affect lease-extension behavior if incorrectly wired, though the underlying DB call remains the same.

Overview
Adds a new heartbeat module introducing a Heartbeater trait plus HeartbeatHandle (real DB-backed lease extension) and NoopHeartbeater (test/no-op), and re-exports these types from lib.rs.

Updates TaskContext to own a cloneable HeartbeatHandle, delegate TaskContext::heartbeat() to it, and expose TaskContext::heartbeat_handle() for passing heartbeating into other helpers.

Changes TaskContext::step so step closures receive StepState<State> (wrapping the user state plus an Arc<dyn Heartbeater>), enabling heartbeats during long-running step execution; tests are updated to use step_state.state.

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[virajmehta]

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