Fix duration enforcement, JSON payload encoding, and error handling in workers

[Copilot]

Workers continued running past the configured duration because in-flight requests were awaited (not cancelled) at the deadline. Additionally, --json always crashed at runtime due to a broken .encode() call on a parsed dict, and worker crashes silently hung the parent process forever.

Root causes & fixes

• Timer not enforced: After the scheduler deadline, asyncio.gather(*pending) waited for all in-flight requests — up to timeout seconds past duration. Fixed by cancelling all pending tasks before gathering:

  # before
  await asyncio.gather(*pending, return_exceptions=True)
  
  # after
  for task in pending:
      task.cancel()
  await asyncio.gather(*pending, return_exceptions=True)

• --json always crashed: json.loads(args.json).encode() called .encode() on a dict, raising AttributeError. Fixed with jsonlib.dumps(jsonlib.loads(args.json)).encode() — validates then re-serialises to bytes.

• Unbounded catch-up burst: When the event loop stalled briefly, next_fire += interval without a bound caused a burst of accumulated missed requests. Capped by resetting next_fire = now + interval when it falls more than one interval behind.

• Worker crash hangs parent: An unhandled exception in bootstrap_worker exited the process without putting anything in the queue, leaving q.get() blocked forever. Added a top-level except Exception in bootstrap_worker that always enqueues an error sentinel.

• q.get() had no timeout: Added a generous timeout (duration + request_timeout + 60 s) with a queue.Empty warning, preventing indefinite hangs even if the sentinel is somehow missed.

• No KeyboardInterrupt handling: Ctrl+C left daemon processes running and printed no report. Wrapped the collection loop to catch interrupts, terminate workers cleanly, and still emit a partial report if data was collected.

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