Fix nest_asyncio uvloop WebSocket import failure

[ruizanthony]

Summary

• Avoid calling nest_asyncio.apply() on active uvloop event loops during WebUI imports.
• Guard both helpers.task_scheduler and helpers.runtime so WebSocket connection setup does not fail with ValueError: Can't patch loop of type <class 'uvloop.Loop'>.
• Add a regression test that imports helpers.task_scheduler inside a uvloop runner.

Verification

• /a0/venv/bin/python3.12 -m pytest tests/test_task_scheduler_uvloop_import.py tests/test_task_scheduler_timezone.py -q → 5 passed
• /a0/venv/bin/python3.12 -m py_compile helpers/task_scheduler.py helpers/runtime.py tests/test_task_scheduler_uvloop_import.py tests/test_task_scheduler_timezone.py
• Live Agent Zero restart verified on local host: /api/health returned HTTP 200.
• Post-restart logs showed accepted /socket.io WebSocket connections and no fresh WS on_connect error, Can't patch loop, uvloop.Loop, or traceback.