fix: avoid blocking Docker readiness checks

[ruirui6946]

Summary

• move the blocking readiness HTTP request off the asyncio event-loop thread
• replace the blocking retry delay with asyncio.sleep
• add a Docker-free async regression test that verifies the event loop progresses during a probe

Why

DockerManager.start_container() could block every coroutine on its event loop for each two-second HTTP probe and one-second retry delay while a container was starting.

Closes #274

Validation

• ruff check utu/env/utils/docker_manager.py tests/env/test_docker_manager_async.py
• ruff format --check utu/env/utils/docker_manager.py tests/env/test_docker_manager_async.py
• pytest -p no:cacheprovider tests/env/test_docker_manager_async.py -q