Fix: AE-1371: poll for jobs while tasks are running

[jhcipar]

Modified from #457

• JobScaler.run_jobs now tracks the active handlers in a set, adds a short wait timeout, and idles with asyncio.sleep(0.1) when there is nothing to do so the loop no longer busy-spins
• Added tests/test_serverless/test_rp_scale.py, a new async test suite that exercises queue draining, concurrency limits, shutdown behavior, and an end-to-end “fetch jobs then process them” workflow using a patched JobScaler.