Fleet-mode live throttle: running workers pick up speed changes

[NitinKumar004]

What this adds

The dashboard's throttle slider now reaches already-running fleet workers, not just solo mode.

Previously a fleet worker created its rate limiter once at start, so changing the speed mid-run only affected new runs. Now each worker re-reads the task's rate between leases and adjusts its bucket, so a speed change from the UI takes effect on the workers currently processing.

Changes

• TaskStore.GetRate — read a task's current rate.
• LeaseWorker refreshes its throttle from the task's rate once per lease (cheap: leases are coarse, ~5 batches each). The rate is already persisted by the existing SetSpeed path, so no new coordination is needed.

Evidence

White-box integration test (TestIntegration_FleetLiveSpeed) asserts a worker adopts a new rate after SetRate.

Live fleet run (1 control plane + 1 worker) while changing the speed via the API mid-run:

throttled @3000/s :  ~3,300 rows/s
--- bump to 60000/s via the slider/API ---
after bump        : ~51,000 rows/s   (the running worker sped up)

Notes

• The change lands on a worker's next lease (leases are coarse), so there's a short, bounded delay — fine for a throttle.
• No UI changes: the speed slider already exists; this just makes it effective in fleet mode.